A/N: For clarification, the mark appears on your dominant wrist (so Ben's would be on his left, since he reached out for the Omnitrix in the first episode of the original series with his left hand).
Rook is three years older than Ben in this fic. I came to this conclusion as notes on the episode 'Clyde Five' says that Clyde has to finish high school (which is when your 18 years old in the US) before joining the Plumber Academy. So we can assume that only legal adults can become Plumbers. However in the Ultimate Alien episode 'Basic Training', Ben and Gwen were able to graduate at the age of 16 (and Kevin at 17). I'm guessing this is because they were already honorary Plumbers (as Kevin and Gwen were given Plumber Badges by Magister Prior Gilhil in 'Darkstar Rising') and they were allowed to take Plumber training early due to them basically being Earth's Plumbers (Magister Gilhil made them the quote "only law enforcement in this quadrant"). However, as Rook is from Revonnah he would have to be of legal age to start Plumber training. It only took Ben, Gwen and Kevin a few weeks to graduate Plumbers Academy, but since they had been 'training' since they were 10/11 I would guess that they wouldn't have had to take the full course. Rook, meanwhile, would probably have had to take the full course which I've narrowed down to being about a year long, so he would have been about 18-20 years old when he graduated depending on when he began Plumber training. Since Rook was in training the same time the Vreedle brothers blew up the Academy (mentioned in 'So Long, and Thanks for All the Smoothies) and took his final exam at around the same time, I'm putting him in the middle range for this estimate. So in this fic, Rook is 19.
It was a surprise for Blonko when he woke up on the morning of his of-age day of birth to find the fur on his right wrist burned away, leaving only a few unfamiliar letters in its place. The responding cry had woken his family and Father took him to the Elders to find a reasonable explanation for the strange occurrence (Young One's answer of 'soulmates' – which he had found on the Extranet – had been quickly put down). When the Elders had come up with nothing, Father, rather reluctantly, took him to Sheriff Wat-Senn to see if the burn had been caused by an offworlder like a Pyronite.
Sheriff Wat-Senn took one look at the burn and had laughed. He then began an extremely long explanation into the 'soulmate' matter that Young One had mentioned, although very little of it made sense to Blonko. Blonko's only response was to ask if the 'soul mark' would inhibit his Plumber training, which Sheriff Wat-Senn assured him would not.
Blonko left for Plumbers Academy the following week.
Plumbers Academy was nothing like Revonnah. For one Blonko (or Rook as he was referred to here) had never seen so many different alien species in one place before. Secondly, the eating habits of many the cadets were less than adequate. He made friends with a Loboan called Scout on the basis that they were both the first of their kind to attempt to become Plumbers. Another cadet, a Lenopan called Mann, was also the first of her species to become a Plumber, but she pulled too many pranks for Rook to even think about befriending her.
One of the many subjects taught at the Academy was on the famous Ben 10. An Earthling who had been given the Omnitrix by Azmuth of the Galvans and used it to save the universe several times. A television series had been made about him that Young One occasionally listened to back home, though as Rook now found out many aspects of the series were fiction.
The first time Rook opened the chapter on Ben 10, his wrist began to burn. He managed to ignore it until he got back to his room before washing it in the sink. It was only then that he realised that he could read the letters burnt into his wrist. This was not very surprising as Plumber Badges had a universal translator built into them so he could understand other languages. What did surprise him was what the letters on his wrist spelt out.
Benjamin Tennyson.
The same name in his text book. The same name as the boy who had saved the universe. The name that was currently on his wrist.
There had to a mistake. From all of the talk Sheriff Wat-Senn had said about 'soul marks' one aspect of them had stayed in Rook's mind: a fated loved one. The one designed by the universe itself to stay with you for eternity. An honour bestowed only on the lowest of sentient beings: humans. Ben 10 must have a name on his wrist, like Rook now had. Out of all the trillions of sentient beings in the universe, the universe had chosen Rook to be Ben 10's. It could not be true. Rook's text book had mentioned that there was over seven billion humans on Earth. Surely there must be another 'Benjamin Tennyson' out there.
Rook pulled his glove back onto his hand and quickly left. He had a class to get to.
Taking his final exam on an asteroid was not the pleasant experience of Rook's life. However he still managed to pass first in his class, albeit with a slight vendetta against those destructive Vreedle brothers.
Originally, Rook's plan had been to graduate from the Plumbers Academy and go back to Revonnah to train as Sheriff Wat-Senn's future replacement, when he eventually retired. Apparently the universe had other ideas.
Magister Maxwell Tennyson. The greatest of all Plumbers, in Rook's opinion (many other cadets had said that Ben 10 was the greatest, but he did not have the experience of his grandfather), who had requested Rook be stationed on Earth to be Ben 10's new partner. This confused Rook greatly. Did Ben 10 not have two partners already? His cousin Gwen Tennyson and her boyfriend Kevin Levin? "Not any more" was Magister Tennyson's reply. Gwen had gone to college early and Kevin had left to be closer to her. Magister Tennyson did not want his grandson to be facing the dangers of the universe alone. He wanted a friend for Ben 10, who was around his age, capable of facing world-threatening dangers alongside Ben 10 for the foreseeable future.
So Magister Tennyson had been watching the new recruits take their final exam. It did not take long for him long to be convinced that Rook was the best possible option for the job. It also did not take long for Rook to accept the station on Earth.
Ben 10 was nothing like the text book, or indeed anything, had said. He was loud, egotistical and obnoxious, thinking himself to be the greatest person alive or dead in the universe. It did not surprise Rook too much. Give a ten-year-old the greatest weapon in the universe, tell him to defeat evil and be celebrated for it is hardly good for the development of modesty. But Ben had his strong points. Despite not being academically smart (his failing grades made such a statement obvious), when it came to strategic planning, Ben was quite possibly the best strategist Rook had ever met. From figuring out where Dr Animo's escape route was from a couple of ants to outwitting Khyber – the self-proclaimed 'greatest huntsman in the universe' – Ben was a leader through and through.
Which was good. Since the Plumber's Academy valued teamwork over independent thinking and Rook was quickly realising that text-book thinking was not going to help him very much in the field.
Ben often wore a hooded jacket despite Bellwood being in the desert, even when visiting Ester. If he was not wearing the jacket, then a long-sleeved shirt hid his wrists where his 'soul mark' would be. Apparently this was common practice for humans who had not met their fated one yet and although the idea of humans keeping some traditions comforted Rook, who had grown up on a planet dictated by tradition, it made it impossible to see if Ben's wrist bore the name 'Rook Blonko'.
The first exposed soul mark Rook saw on Earth besides his own and glimpses of strangers on the street was that of Gwendolyn Tennyson. Admittedly, Gwendolyn was a lot younger than Rook had expected to be as someone with a soul mark, but Gwendolyn had responded with an explanation that only male humans got their soul marks at 18 whilst females got theirs at 16. This made sense as Rook had never seen a soul mark on Ben (not that he had had chances to check). Either way, with Ben and Kevin busy distracting the Tetramand Princess, this left Rook and Gwendolyn together a lot, during which Gwendolyn often complained about her boyfriend's apparent infidelity.
"Forgive me if I am wrong but did Kevin Levin not betroth himself to the Princess before becoming committed to you?" was Rook's response to the matter.
Gwendolyn stared at her exposed soul mark (which bore the name 'Kevin Levin') for a while before sighing and admitting that she was probably being too harsh on Kevin. When the two left, Ben congratulated Rook on keeping his cousin and her boyfriend together as it was unhealthy for soulmates to be apart for long periods of time and breaking up usually ended much of the contact between soulmates.
Rook gave himself a 'mental note', as humans say, to research soulmates and soul marks for more information. He would of asked Ben but his partner was quickly distracted by the thought of smoothies and the impending problem of now being betrothed to an alien princess.
Rook could not lie and say that the thought of Ben being betrothed to anyone unwillingly made him uncomfortable.
Remembering back to when Magister Tennyson had said that it would be beneficial for Ben to understand other traditions besides human ones, Rook should not have proposed that Ben visiting Revonnah during the harvest be his first lesson. Revonnahganders were very particular in how their traditions were performed and the Harvest Festival was the most important of them all. If not enough Amber Ogia was harvested then Rook's people could starve.
However, Rook was getting 'homesick' and wished to participate in the harvest again (something that he had not done since leaving Revonnah to become a Plumber). In all fairness to Ben, his partner did try to follow tradition in the way that Rook told him to, but having never done it before – or watched anybody else harvest before him – he was doing a poor job. Such matters were made worse when Fistrick began to steal Amber Ogia. Father had a low opinion of humans already since his oldest son was fated to be mated to one of them instead of a nice Revonnahgander, and Ben's poor performance at the harvest along with Fistrick's thievery made it so that eventually getting permission from Father to marry a human would be difficult. Especially since so many Revonnahgander females had asked for his hand in marriage whilst he was away, their admittedly beautiful neighbour Rayona being the first.
Mother had taught Rook long ago that lying to oneself was unhealthy for the mind. So Rook did not lie to himself that if the name 'Benjamin Tennyson' had not been burned onto his wrist he would of considered Rayona a viable option to being his future wife. However, such a name was burned onto his wrist and Rook's slowly waning trust in the universe to make good judgements kept him from taking Rayona's proposition to attending the Harvest Festival with her. Instead, Rook stayed with his family and Ben, staring wistfully at the fireworks in between laughing at Ben's unfortunate attire.
Admittedly, sending the most powerful being in the universe away whilst his home world was being conquered was not the most intelligent decision that Magister Tennyson had done. Also considering that the most powerful being the universe had over half a decade of training and had dealt with more terrifying beings before the Incurseans made the decision even more stupefying.
Rook was one of the few Plumbers to escape the Incurseans and imprisonment, Blukic being another. Most of the other Plumbers who had escaped were quickly recaptured. They had not the months of evasive field training nor the genius-level intellect that Rook and Blukic had respectfully. Although even Blukic would admit that joining up with Kevin and Gwendolyn was a tactical advantage. Argit was less so an advantage as he was a hindrance but Kevin spoke out in favour of him and his poisonous quills so Rook allowed the rat-like being to help.
He wished he had listened to his 'gut instinct' on that decision.
Being saved by an Incursean was not the most boastful thing Rook had ever done but it was more favourable than the alternative of locked up with no hope of escape. Additionally, there was something about Bullfrag that made Rook instinctually trust him. Maybe it was the obnoxious attitude, workable tactics or some unknown universal force but Bullfrag reminded Rook of Ben. It was therefore not a great surprise to Rook that Bullfrag turned out to Ben.
What did surprise Rook was the euphoric joy he felt within when he was properly reunited with his partner at Mr Smoothies. It was as though the missing piece of himself that had been surgically removed had been returned, albeit not returned within his body but within a box, figuratively speaking. Rook thought that he was successful in keeping this unfamiliar feeling to himself, but Gwendolyn had noticed.
Admittedly, trying to keep a force that tied two lives together a secret from a being that could see life energy was not the most intelligent decision Rook had ever made. Also considering that very same being was also the cousin of his possible soulmate and had spent over a decade and a half being raised alongside that cousin made the decision even more stupefying.
The following conversation between Rook Blonko and Gwendolyn Tennyson was vaguely familiar to the conversation the two had had a few months ago.
"Wouldn't you have known about this ages ago? I mean, you're older than Ben. You must have had it a while."
Indeed some part of Rook had known that the Bearer of the Omnitrix was his soulmate since his of-age birthday. Even before finding out what the writing said Rook had held Ben 10 in high regards as someone to aspire to be. But why would the most powerful being in the universe consider him, a lowly Revonnahgander who hadn't been in the field for much longer than half a year, as a life-partner?
Gwendolyn had responded with questioning why one of the most powerful magic-users in the galaxy would choice an ex-convict as a life-partner.
Looking beyond Gwendolyn's choices in boyfriends, Rook could understand what Gwendolyn was referring to. There was no reason in trying to outcross fate, especially when proof of such concepts were real and as tangible as the writing on one's wrist.
Rook rarely lied to himself. It did the mind no good to do so. However it now appeared that Rook had been lying to himself for well over a year. It was time to stop.
Researching human courting conventions was a lot stranger than Rook had previously anticipated. Beyond the mere differences between human and Revonnahgander courting rituals, there were so many different ways to show one's affection to another.
On Revonnah, courting traditionally consisted of presenting the receiver with a freshly baked good, commonly a pie, and then asked them to attend the next festival together. Occasionally, permission for such things was given by the parents of the receiver if the receiver was busy or away somewhere, as had been the case with him and Rayona, but such things were uncommon at best.
Human courting was more commercialised with many websites suggesting a specific flower to be given or hand-to-hand contact, neither of which Rook saw the significance of or would help him in such a situation as he was in. Ben and he often made physical contact with each other – it was part of being a Plumber to sometimes having to hold hands when fleeing from a criminal – and Kevin claimed that flowers were for males to give to females, not males to other males. Although Gwendolyn had slapped him on the head for that comment so Rook was unsure of how reliable that advice was.
Luckily, Gwendolyn was happy to give useful advice on courting Ben, indeed she was overjoyed as Ben had been the one to bring Gwendolyn and Kevin together before Gwendolyn's soul mark had appeared. Magister Tennyson was also quick to offer his advice, although it more about what Ben's preferred flavour of pie was than ways of courtship. Max, as Magister Tennyson insisted on being called, had never had a soul mark. The mother of his children – he hesitated to call Verdona his wife – and he were never fated but that had never stopped them being together for the brief amount of time that they were. Although their relationship had never lasted beyond the growth of their sons, Max said that they had a psychic connection to each other very similar to that of mated soulmates so maybe they were fated but the universe had neglected to place a soul mark upon their skin.
Maybe the advice of making apple pie was the best way of starting this unusual courtship.
Ben was quite enthusiastic upon receiving the apple pie and had asked what Rook's favourite flavour was. Rook was unsure if Ben understood what the pie represented but he was thankful that Ben liked his baking more than his cooking.
Ben's mother, on the other hand, knew exactly what the pie represented and was quite accepting of Rook courting her son when Rook had shown the soul mark to her. Ben's father was not quite as accepting and had threatened bodily harm upon Rook with his many blades if he harmed Ben. It concerned Rook that Mr Tennyson had apparently tried to get the Omnitrix off his son's wrist with those same blades, or at least ones similar to them, but Mrs Tennyson had insisted that Ben was fine after the ordeal. And honestly, Ben had been through much worse than a failed attempt at removing the Omnitrix by his father.
It then occurred to Rook that if Ben was left-handed and the soul mark appeared on the dominant hand of a person, then the Omnitrix may be obstructing the formation of a soul mark on Ben. Although Ben was not even seventeen yet and had a little over a year for a soul mark to form, it worried Rook that Ben may not believe him if a soul mark did not form on Ben's wrist as it had on Rook's. This was something to ask Azmuth about the next time the two crossed paths.
As it turns out, meeting Azmuth again was not too difficult. Trying to speak to Azmuth alone without Ben overhearing was more difficult.
After saving the greatest mind in the universe from having his mind lost to his former assistant, Azmuth had immediately teleported back to Galvan Prime after rewiring Albedo's Ultramatrix to trap the Galvan in the form of an 11-year-old Ben. It did not take Rook long to convince Max that he had to speak to Azmuth regarding the Omnitrix obstructing the formation of a soul mark. Azmuth was originally unconcerned about the theory, saying that he would 'get around to it' when the matter arose on Ben's 18th birthday, but Max saying that Rook was Ben's fated one made Azmuth at least interested in the matter.
"A human being fated to a Revonnahgander? A very interesting predicament."
Rook managed to get Azmuth to agree to not say anything about the soul mark to Ben (which Azmuth found very amusing, even putting in his own suggestion of just telling Ben that the two partners were fated) and took many pictures, x-rays and blood tests of Rook's soul mark only the declare it as real and as effective as any human soul mark. He could not do much about Ben's eventual soul mark before it appeared, although there was a remote possibility of it not appearing due to Ben changing forms so often.
At least Azmuth said that if Ben did not believe his partner, cousin, grandfather or parents about the trustworthiness of Rook's soul mark that the test results would ensure scientific reliability. Although Ben rarely considered science to a part of anything beyond a classroom and the results would probably be unconsidered if such a problem would occur.
Despite everything that Kevin had said about males not giving flowers to other males, Rook decided that since so many human websites had declared the gesture effective it was worth an attempt. Although giving Ben an apple pie had not worked in getting Ben to realise Rook's affections for the human, Rook had decided that it was because such a gesture was more popular on Revonnah than on Earth so using an Earth custom would be much more likely to work.
After acquiring the flowers, a bouquet of red roses was most effective the websites had said, Rook gave them to Ben the next time they had to do patrol.
Although Ben had taken the flowers from Rook, there was no giggling or blushing from him as the internet had said there would be, more of a bemused expression before Ben had responded to Rook's gift with, "You know, usually people give flowers to their partner's mom when they first meet her instead of to their partner. Do you want me to give this to my mom, or do you want to give it to her instead?"
Rook thought that since Ben had accepted the flowers that meant that he had accepted Rook's proposition of courting him. This was quickly put down by Mrs Tennyson who said that it was common for co-workers to give each other's parents flowers to gain their trust.
It seems as though another tactic would have to be used to gain Ben's permission.
As interesting as it was to be in the Anur Transyl, Rook would have appreciated visiting Scout's home planet more if Lord Transyl had not taken over his body and forced him to fight his fated one. The whole trip had become a failed mission to get the body of Dr Viktor back to Anur Transyl and now every time Rook laid a harmful hand on his fated one his soul mark burned painfully. It had long since burned through his Proto Armour (something that Rook did not think was possible) and it felt as though the soul mark was burning its way into his bone.
Upon being freed from Lord Transyl's control, Scout had wrapped up the soul mark using some supplies he had at his base to keep the soul mark a secret from Ben and Rook explained the situation to his friend. Scout had originally been confused about why Rook would keep such a secret from his fated, although the knowledge that Ben was likely to reject Rook before receiving his own soul mark was enough to keep the Loboan quiet.
Keeping in contact with his cadet friend was near to impossible for Rook to do due to Scout's lack of access to the Extranet. Leaving was painful but the citizens of Anur Transyl were slowly becoming more accepting of coming into contact with those outside of their system so Rook doubted that he would be out of contact with his friend for much longer.
Despite what the Plumbers (including Blukic and Driba) thought, Rook in fact had some memories of his time under Fistrick's 'bro essence'. He did not particularly like being under its control, particularly when he participated in an illegal fight club, which Rook considered to be a stain on his personal honour. However, the events did leave Rook without his inhibitions which gave him the confidence to initiate another of the courting tactics he found online: hand-to-hand contact.
Unfortunately, Ben did not see the unnecessary touches as being anything more than the effects of the 'bro essence' so the admittedly embarrassing events did nothing for Rook in terms of advancing the relationship.
At least that is one more tactic proven to be useless. And giving the Golden Fist into Plumber processing meant that Rook did not have a stain on his conscious.
Rook knew from the moment he decided to willingly attempt to court Ben that there would be some, particularly on his home planet, that would not be accepting of his choice of mate. His former Revonnah Kai teacher Master Kundo being 'top of the list' so to speak.
Master Kundo hated anything technological. Even Rook's Proto Tool, which had saved his people's way of life back when Fistrick tried to steal Amber Ogia from them, was considered to be 'dirty' and 'wrong'. It had taken many attempts from the other Elders to convince Kundo that the silos used to store Amber Ogia were beneficial towards Revonnah's way of life as opposed to storing it in caves, basements or even underground where Muroids could take it.
That being said, Rook had never wished for his former teacher's fate to be dissolved in acidic water. It was a horrible and yet fitting demise for someone who hated advancement so much that he would resort to hypocrisy to kidnap three of his own.
Although that day did have a 'silver lining' with Rook's brother losing his bi'nthak and becoming an accepted Revonnahgander. The effect Ben had on his family was obvious when his brother chose 'Ben' to be his name. Although Rook wondered what name Ben would have to take if they were to ever become betrothed as there was already confusion within his family when referring to a 'Ben'.
Never had Rook been so thankful to the universe when Ben insisted on bring him along on his 'date' with Ester. Although he could have done without the thieves and visits from a mysterious boy who was quite obviously not Jimmy Jones now.
The mysterious boy seemed very particular in trapping him and Ben together, being distraught about Kai and Ben being the ones trapped and how "it should've been you". He also seemed very happy that Kai and Ben had spent their time attempting to escape their prison rather than talking to each other about anything other than escaping and finding the thieves.
Rook had to admit that he was also grateful that Ben felt no attraction to Kai as he had when he had been ten years old.
Meeting the future version of Ben, who called himself Ben 10,000, had been…interesting to say the least. Figuring out the concept of time was difficult to someone who was used to only going forwards through it. This was not an issue for Ben, though, who was used to the concept of time travel, alternate dimensions and also 'had a mind for theoretical physics' which was saying something considering Ben was currently failing physics in school.
Ben 10,000 did give Rook some hope for the future by winking at him. According to Gwendolyn, winking meant one should not lose hope in what they are currently aspiring to do. Especially when the one doing the winking comes from the future.
Watching the various females Ben had met since acquiring the Omnitrix compete for his hand in marriage was discomforting for Rook to witness. Especially when Kai was declared the winner and Ben kissed her.
Rook was eventually confronted about it by Max whilst he was in a depressive state, saying that Ben had accidentally kissed Kai and that neither liked each other in a romantic (or indeed very friendly) way. Besides, Kai's soul mark did not have the name 'Benjamin Tennyson' on it. Instead, a random male's name whom Kai had yet to meet was burned on her wrist.
Despite the revelation that his fated one's past crush would never become his wife, Rook was still 'down in the dumps' about his relationship with Ben. If Max could love someone whose name was not on his wrist, then why should Ben? Especially when there were so many attractive females available for Ben to get romantically involved with.
It was at this point, that Rook decided that Ben 10,000 had been wrong. Maybe the Ben 10,000 they had met was from an alternate future to theirs, one where Ben actually reciprocated Rook's feelings for him.
As it turned out, the mysterious boy that Rook and Ben kept meeting - Spanner - thought differently about Rook's theory. Time travel is a strange concept to experience and being sent into the future with Ben by Spanner was no different to being trapped in a time loop like the previous week.
To the untrained eye, it would appear that Ben 10,000 and Kai were married with a son, who turned out to be Spanner. Although Ben later said that if Ben 10,000 and Kai were married then their marriage must not have been a good one since the two were abrupt and cynical to each other. They were more like unwilling partners in a business that required them to work together than a happily married couple.
Unfortunately, Rook could not see Ben 10,000 soul mark due to the Biomnitrix so Rook could not say that Rook was indeed Ben's fated as Ben was Rook's. But the winking Ben 10,000, Kai, Gwendolyn, Kevin and even future Max gave him renewed some of the hope Rook had that Ben loved him too.
After all was said and done it took a literal push from Spanner for Ben to confront Rook about the soul mark.
It began with a statement, rather than a question like those found on the romance websites. "You know, Spanner said that you had a soul mark."
Rook looked up from the smoothie he was currently drinking. "Indeed I do." Ending it like that was Earth custom for saying that the conversation was over. Rook's heart was already in figurative pieces, there was no reason to break it even further.
Ben, apparently, was either oblivious to this custom, or more likely wanted answers that he was not getting. "I know this is private thing, but I thought that soul marks was only for humans and you aren't human."
"I am aware."
"Can I see it?"
Rook stared at his drink for about two minutes before rolling the sleeve up on his wrist to reveal the name 'Benjamin Tennyson' to the one the universe said was to be his fated one.
Ben looked at it with a quizzing face, as though he did not believe that Rook's soul mark was real. Rook eventually decided to 'press his luck' at convincing Ben. "If you do not believe it to be real then your grandfather, cousin, Kevin and Azmuth will say it to be true."
"You talked to all those people about this?" The voice was pained.
"Yes. Does it upset you that I did?"
Ben looked at the Omnitrix. No, he was not looking at the device that changed his life. He was looking at his dominant wrist. The one that may one day bear Rook's name upon it.
Rook continued with his story, taking Ben's silence as Ben being upset about him talking to his family and mentor about his soul mark. "I tried to court you according to human and Revonnahgander custom. I gave you an apple pie, red roses and initiated hand-holding. You did not respond to them in any way that said that you were 'interested' in courtship."
His loved one then grinned and gave a familiar wink to Rook. One that Rook had seen before and in the future.
"You know, if you wanted to ask me out you should've just said so."
A/N: I know I'm leaving it abruptly where it was and it may be difficult to understand when each line break ends off time-wise according to the Omniverse TV series, particularly if you haven't watched all the episodes (at the time of writing this I haven't watched the final four episodes and likely I'm not going to as I've read the plot and it doesn't really interest me) so it goes like this:
1/2/3: Before the Omniverse series begins. Technically this is during Ultimate Alien. Continuity between these two series is difficult, especially when taking into consideration the original series (like when Ben meets Albedo for the first time when he's 11 but doesn't recognise the name when he's 15) so getting a straight time reference is difficult but since Rook is at the Plumbers Academy at the same time as Octagon and Rhomboid Vreedle it's safe to say it's after 'The Mother of All Vreedles' which is when the Vreedle Brothers decide to become Plumbers.
4: Between 'Of Predators and Prey: Part 2' and 'Many Happy Returns'. I skip over most of Season 1 as it's mainly about Ben and Rook getting used to each other and finding each other's strengths and weaknesses so they can work together as partners.
5: 'Many Happy Returns'. Yes, Ben does get engaged to Looma at this time but since Rook is still in denial he isn't too concerned about Ben getting unwillingly engaged to an alien princess.
6: 'Bros in Space'. Rayona does ask Rook to the Harvest Festival, but Rook denies her despite his parents' prompting. This is part of the reason why Ben doesn't go after Rook by himself since he thinks Rook has a girlfriend in Rayona.
7: 'The Frogs of War: Part 2'
8/9/10: Sometime after 'The Frogs of War: Part 2'.
11/12: Sometime after 'For a Few Brains More'.
13: 'The Vampire Strikes Back'.
14: 'No Honor Among Bros'.
15: 'Rook Tales'. From here on out I take some liberties with the canon of the show.
16: 'Fight at the Museum'. Ben and Kai are still trapped in the museum by Spanner, but Spanner was trying to trap Ben and Rook together but failed.
17: 'Let's Do the Time War Again'.
18: 'The Most Dangerous Game'. I have only read the plot online and have not seen the episode itself. I've never found romantic shows interesting or entertaining and this definitely falls under my category of 'potentially uncomfortable for me to watch'.
19: After 'A New Dawn'. This is just before Ben decides to go on a 'road trip' across the universe with Rook, Gwen and Kevin. Realistically, there would be no way these four could see the whole universe during their lifetime. But this is a show that centres around a teenage boy turning into different species of alien, one of which is a single-celled organism (Goop) which shouldn't be biologically possible so it's safe to say that the show takes some liberties with science. Personally, I think that maybe they could have done a couple of galaxies considering the technology the Plumbers have, but even within the context of the show it seems that if Ben leaves Earth for longer than a week a new conqueror tries to take it over so it wouldn't be good for him to leave for too long.
I'm planning to do a second part to this fic about the aftermath of Rook and Ben getting together. If I do, it will take place two years from when Omniverse ends and would end at around the time of 'A New Dawn' on the future side of the events.