I did mention Kaguya-sama characters may eventually show up. I did my best to match up the timelines roughly based on all the evidence, though. My analysis of the timeline goes roughly by sweet today, which is a series that shows up in both. While in Kaguya-sama, it was mostly in manga form, it was nearing completion. In Oshi no Ko, it's done and is receiving an adaptation. I can't imagine something taking super super long to get an adaptation after its done, especially since the author still looks pretty young, not to mention something that seems pretty popular, at least with the student council. Kaguya also makes an appearance as a photographer of Ruby, but she doesn't seem much older than the series end, although she is already married. It could be a situation of Asian youth or whatever, but I put the timeline as Kaguya and Miyuki getting married roughly immediately after college, assuming they did the whole 4 years, or maybe they did prodigy stuff and graduated earlier. Nevertheless, my guess is that they're married around 22-23 years old ish, somewhat near the time Kaguya started taking pictures of Ruby who was roughly 17ish at the time to my knowledge. So about a 5-6ish year difference between them.
Following the timeline that way, Kei is 3 years younger than Miyuki and is thus 1 years older than Kana and 2 years older than Aqua, Ruby, and Mana. Miyuki is 4 years older than Kana and 5 years older than Aqua, Ruby and Mana.
EDIT as of May 27: I just realized news that high school Ruby probably had a year or so before Kaguya took her picture. Reorganizing the ages for consistency (and also so that the age gap is more accurate and also a bit closer, a bit more convenient)
Saitou Miyako, pretend mother and caretaker of the twins, stumbled slightly with how lost in thought she was as she accompanied the Hoshino trio home.
The playdate was… interesting to say the least.
Her first impression of the family was honestly somewhat negative. Somehow, the younger child ran forward and escaped his mother, reaching them before the mother-daughter duo could catch up. Personally, Miyako was flabbergasted. The child was three and running about on his own.
The mother should have paid more attention, Miyako felt. A careless mother who didn't appreciate the life she had and would soon meet tragedy. Miyako judged the Arima mother privately, although she knew very much better than to voice her concerns.
Moving on to her impression of the child who had run away and ditched his mother, that kid seemed like he had an excuse for everything, too. The epitome of an annoying, spoiled brat. Miyako felt blessed to have Ruby and Aqua instead of Kana and Mana. Those two twins never did anything like that, always being relatively well behaved.
'Literally blessed.' she commented in her head, eyes flashing as she recalled that day. Aqua's cold eyes on the table as he spoke, the ghost of divinity judging her as she sought to upend everything Hoshino Ai fought for. Ruby's anger, the conduct of the god Amaterasu herself being channeled to slap away the hand Miyako had extended toward her.
Well, she supposed that even the twins had their strange sides, Miyako thought. Although, and she wondered this rather loosely, how much of their behavior came from the influence of the divine?
Ruby would sometimes fangirl energetically, dancing and raving, obsessed and disgustingly otaku-like, while Aqua had a somewhat gross gaze while watching their mother dance on screen, his gaze so filled with admiration for his mother as an idol that it honestly made Miyako uncomfortable. The twins didn't make too much of an effort to hide their admiration for their mother and idol after the reveal to her, and so Miyako was forced to watch the kids act as disgusting otakus in private whenever Ichigo and Ai were out for work.
If there was anything that convinced her that they were touched by divinity, it was that, to be honest. Of course, Miyako had already been convinced by the twins before. Children did not speak full sentences, comprehend complex concepts, even plot to increase the popularity of their mother, an idol. However, it was the sheer passion and admiration the twins had for the idol that struck her.
It always struck her, how inhuman it was that two babies were speaking fully like grown humans, answering and plotting in every way to make Ai happier, stronger, and better. It was so out of the natural order of humanity that Miyako could barely wrap her head around it.
But she never really realized how insane all of that was. How insane it was to be near those who have been touched by divinity. Not until today's playdate.
The playdate forced her to essentially watch kids play for some long hours. Those hours were long in so many ways. She had to keep track of Ai, make sure she wasn't getting into trouble. Even while she was a caretaker, she was still Ai's manager, after all. She also had to make sure to keep up the image of, not just being a good manager, but also being a good mother. She's had a lot of practice over the years of being the kids' caretakers, but now she was in close contact with another person, a mother herself. She had to keep her guard up.
And those considerations were only her personal circumstances, distracting her. It was the kids, innocently playing in the playdate, that really got her thinking.
Miyako isn't a stranger to children. While she was an only child herself, well, she worked in the entertainment industry. She saw tons of children enter and exit the entertainment industry. A lot of them were precocious, a lot of them were annoying little brats, but honestly, they were just annoying little kids that she saw from a distance, no one she was actually involved with.
But watching the playdate set off so many conflicting feelings from within her. The Arima siblings, Mana and Kana, well, they looked to her like innocent little kids. Running around, being all around brats, disobedient, noisy, annoying. The contrast of them to her image of Ruby and Aqua highlighted the inhumanity of Ruby and Aqua's private behavior.
But what really surprised and confused her on the playdate was how Ruby, and even Aqua seemed to have become children in front of her eyes.
The Arima brother poked Aqua in the cheek and said "Tag, you're it!" and ran away, and suddenly, Ruby and Aqua became kids again. They were running around, laughing, having fun, and for the first time since that fateful day where she found influence of the divine, she felt like she was seeing them as kids again.
It was conflicting, it didn't make sense. They were inhuman fans of Ai, entities touched by the divine to support Hoshino Ai. They were monsters in their own way. Monsters who she eagerly sought to be on the side of, else their prophecy of her death become something that turned true. Monsters who she fought to be on the side of, just so that she could continue to have a bright future.
But here, on this playdate, they just seemed like… happy kids. They had their quirks, they had their subtleties, they had their complaints, they had their own little parts of them that made them happy little children playing with happy little children.
Her first thought was, well 'It's a lie.' Because how could it not be? How could the inhuman divine entities show such human behavior? But all of that was blown away the longer she stared at the kids having so much fun.
Aqua's eyes lit up in glee as Kana glanced over his hiding spot before turning away to search for other places.
Ruby's smile and laughter as she beat Mana in a race, even if she was only 3rd fastest among the group of four.
Aqua teaming up with Kana to surprise Mana who thought he hadn't been found.
Ruby grinning wildly as she argued with Kana over the rules.
In the moment of the playdate, they weren't divine entities. They weren't monsters. They were simply children, cute children who Miyako had been part of raising.
Miyako's eyes softened.
'Divine entity. Monster. Demon. Human child. Whatever and whichever you are, be it one or all of the above.' Miyako listed out, the feelings in her heart conflicted, and yet she knew there was only one way forward for her from then on. 'I will be on your side. Because even if I am not your flesh and blood-' Miyako couldn't help but glance at Ai, who was busy pampering Ruby at the daughter's request, and Aqua, who seemed to be watching them and mumbling to himself.
'I am still your caretaker. I am the one who is always by your side. I am the one you have trusted with the secret of your divinity. To me, you are my son and my daughter.'
Something shifted into place, deep within her, and satisfied, Miyako patted Aqua's head. Aqua looked at her, surprised, but not suspicious, and stayed in place for her.
For the rest of the day, Miyako's smile was a little softer and warmer.
"Kei-nee!" the silver haired maiden turned around as she was walking home from school to see the neighbor's 3 year old kid running at her, leaping at her.
She was a good 5 year old girl, so she successfully caught him and held him. Her father beside her was surprised at the sudden charge but not overly shocked. The kid had done this before, after all. The child laughed with glee in her arms before she slowly and carefully placed him back down onto the paved road.
This kid has always been super excited to see her, so she always tried to wake up and prepare for school at least a little early so she could greet her. She's a good kid, just like her big brother. All of that prep just so she could handle talking with the cheery kid and his sister before school.
The kid's pretty cute and very nice, the 5 year old girl thought. She's 3 years his elder and was dubbed his older sister. He seemed to have gotten attached to her since the day their mother introduced him to her almost a year prior, a simple greeting the neighbors sort of ordeal.
As a favor to their mom, dad would sometimes take care of the boy and the boy's sister when the mom gets a little too busy. Two siblings were usually so nice and they played with her a lot (especially when her brother was at school), and their mom always made sure to bring lots of food every time. Their mom was really nice, and sometimes she couldn't help but wish that she would come over more often. Her mom rarely came home anymore, so it would be nice if the Arima mother would come instead.
The female of the siblings, Arima Kana, was mostly a reader. She kept reading the movie scripts and practicing her lines. She was hardworking, the silver haired lady thought. Her older brother really seemed to like her, treating her just as well as he treated his own flesh and blood. She wasn't really sure, but she thought that Kana working so hard was super cool.
The male of the pair of siblings, Arima Mana was her preferred sibling, however. And how could she not like him? He adored her, pretty much from when they first met. The child was cute and polite, and he called her 'Kei-nee'. That's all the 5 year old silver haired maiden needed. She would be a good role model just like her older brother is.
The silver haired maiden smiled, absentmindedly ruffling the male child actor's hair, who leaned into her touch.
Behind him, she noticed the mom finally catching up to the kid who'd burst out running towards the neighbor she was so attached to. "Good afternoon, Miura-san. Hi, Kana-chan." she greeted Mana's mom and sister parroting her dad's greeting, familiar enough with her neighbors that she greeted them with their first names.
"Kei-chan! Thank goodness you were able to catch him. Mana just ran towards you the instant he saw you!" the mom complained, her annoyed gaze poking and prodding at her youngest son. In response, the son looked sheepish but smiled unrepentantly.
"But mama…" the child elongated the last syllable in his pout. "Kei-nee always catches me! It was Miyuki-nii who dropped me that one time." he attempted to reassure his mother. The silver haired maiden wasn't convinced and it looked like the mom wasn't either. The mother sighed and shook her head.
Kei thought she should join in on the scolding too, though. What if she didn't notice him in time and they both fell? She was about to join in when the Arima elder sister got mad at him. "Mana, you did it again! You just did this a while ago! Why did you do it again?!" Arima Kana complained, her face getting redder and redder.
"Sorry, nee-chan…" the child apologized, but the 5 year old knew him. She got the feeling he would probably do it again anyway. The elder sister pointed angrily at the silver haired maiden. "Sorry, Kei-nee…"
The mother sighed when hearing his response. "Thank you very much for your help, Kana… You too, Kei-chan." the mom frowned, clearly familiar with Mana's antics, before turning to her neighbors before her. "I hope Mana wasn't bothering you?" she asked politely.
"Oh no, of course not!" Her dad immediately denied, this time being the one to ruffling the child actor's hair. Mana smiled at Kei's father, turning to grab the silver haired girl's hand after the father neighbor let go. The child looked one last time at his mother, a questioning look in his eyes, and receiving a nod in return. He smiled brightly and turned back to his neighbors.
"Kei-nee, Kei-nee! The truth is, recently, I was filming for a movie! The casting was finally publicly announced, so I can tell you now that I'm gonna be playing a role in it!" The 3 year old looked so happy, and she couldn't help but smile with the boy. "Kana-nee and I have even made three new friends!" Mana continued as they walked along the road that led towards the area they lived in.
"So that's why you've been going out so often!" As the neighbor who was often visited by Mana who was very attached to her, she did notice that they seemed to be going out more often. At first, she thought it was Kana's acting career, which was actually true, Kana is a genius child actor, but she didn't know that Mana was also becoming an actor and even got a movie role too.
"Congratulations!" her dad said, and she followed. She meant the congratulations wholeheartedly, feeling almost like his real elder sister.
Mana grinned happily at her congratulations. "Thank you, Kei-nee!" He sped in front of her, grabbing both of her hands. "Kei-nee, Shirogane-papa would it be okay if we could watch the movie together? Let's invite Miyuki-nii too!"
He leaned past her to give puppy eyes to his mother, who was walking behind them and watching them interact. "We can do that, mama, right?"
Kei couldn't see his eyes from this angle, but she knew he was giving his mother the puppy eyes. The silver haired maiden has experienced them many times. She shared an exasperated look with the elder Arima sibling.
It was only a matter of time before the mother gave in. Not that the mother was planning on saying no, anyway. She was going to invite them herself. "Of course we can do it, but only if we're not bothering the Shirogane family, okay?" the mother put a condition, but Kei knew her brother and father would agree.
Those two loved the Arima kids almost as much, if not more than she did, after all. And both of the kids were going to show up in the movie. She and her family would definitely support her friends. Kei glanced at her dad, waiting for his decision. Unsurprisingly, his response was as expected.
And that's our first (two) Kaguya-sama characters incoming! The Shirogane family are neighbors to the Arima family and often babysit for them and vice versa.
Truth is, I first imagined the Kaguya-sama characters being high school already, and then Mana would worm into their hearts using the ultimate power of friendship and everything else that actually works, but then age-wise, it just doesn't match up with the timeline. I'm not a super stickler for matching the timeline, but 6 years age difference is pretty big...
I'll stick with the Oshi no Ko fandom and not really write this as a crossover fic in , though. The focus is still on Oshi no ko, not just in characters, but also in concept, situation, and in other similar themes.
Also, I really appreciate all the kind and supportive reviews and feedback that everyone has been giving. It has just always bothered me, as a fan of both isekais and SIs, that people don't really mention or consider the moral or ethical boundaries that being reincarnated entails. I love that people appreciate the different tone to the usual SIs that this fic is aiming to move toward. Thanks for reading and being here.