Leviathan

At five, he became a monster.

A dark-haired child ran through the ship's vast corridors. He looked to be around five years old, had hair that went down to the base of his neck and was wearing a red T-shirt with jean shorts. He seemed to be looking around, looking for something or someone.

"Hey! Ise, did you find him?"

Another boy, one that was a little bit older, looking to be around eight years old, and relatively similar in appearance to Issei, but with pink hair instead, asked the younger one. He was Monkey D. Lucy, Luffy and Rebecca's son.

"No. I bet he's cheating again!"

Issei said, with an annoyed face.

"Shishishi."

Suddenly, they heard laughter echoing down the hallway. Said laughter made veins pop out on the boys' foreheads.

"HE'S MAKING FUN OF US!"

They spoke at the same time, and instantly went running off to find their father, both wanting to win their game of hide and seek.

They went looking through various areas of the ship, bedrooms, hallways, the aquarium, the library, the kitchen and so on.

"They are as energetic as ever."

A young man who looked to be in his seventeen years commented, not taking his eyes off his book. He had a lightly muscled body, a trained appearance and had dark hair as well, though his was much shorter than Issei's. That was Monkey D. Nico Saul, Robin's and Luffy first son.

"But that's where the fun is, isn't it?"

Said a red-haired girl who looked to be only a year or two younger than Saul. She said that at the same time she was drawing a rough map on the table. That was Monkey D. Mikan, Nami and Luffy's first daughter.

A smile formed at the corner of Saul's mouth after he heard his sister's words.

"Yeah, it really is."

While the two talked, the little ones continued the search for their father. Every now and then they could hear him chuckling, showing that he was having fun with their fruitless efforts.

"Ise, I'll go this way and you go that way!"

Lucy said to his little brother, pointing out the directions they should go. Both were on the limit of their patience and wanted to find Luffy to bring an end to the game.

"Ok!"

Believing that the chances of winning would be increased this way, Issei does as his brother suggested.

He passed through several corridors, looking cautiously between them, entering various rooms, looking in every possible corner, and always coming back empty handed.

"Argh... even after learning haki it's still hard to find dad!"

Like all the children in the crew, Issei already had his training in haki, and in fighting in general, started some time ago. He had an affinity for observation haki, but even so he could not sense Luffy's presence.

Currently, he found himself in a room with a few books. It was probably a study room for some of the crew members.

The little boy stopped for a bit, looked from side to side. Noticing he was alone, a goofy smile formed on his lips.

"Hehe, I bet uncle Brook or uncle Sanji are hiding something here."

Perhaps it was unavoidable, but the cook and the musician ended up having some influence on Issei. Several times he heard them talking about, and sometimes they even spoke directly to him, about "the wonders of life" and about "a man's romances". The fact that he was also pampered by the inhabitants of Amazon Lily might also have had some factor in his affection for the opposite sex.

Needless to say, when not only Hancock, but most of Luffy's other wives found out about the two's influence on Issei, the result wasn't very positive for either of them.

However, what was done was done.

Issei was momentarily distracted from the game, and tried looking for a specific type of magazine.

"Looking for something?"

A female voice reached his ears. Gulping, turned to see one of his older sisters, Monkey D. Jewerly Jeanne, Bonney and Luffy's daughter. As Lucy, she was also pink haired. She seemed to be of the same age range as Mikan, and was looking at him while holding back her laughter.

"N-nothing."

The little one said, quickly leaving the room. Jeanne finally left the laughter out of her throat; they could hear her throughout the whole ship.

"OK, where do I look now..."

It had passed quite some time, and he still hadn't even seen a shadow of Luffy. Issei was no longer paying attention to where he was going anymore and this resulted in him crashing headfirst into some metal surface.

Taking a few steps back the boy noticed that he had arrived at the end of another hallway and had come face to face with a metal door, which had the top of it the words "Vegapunk's Laboratory".

The younger children were usually kept away from the lab, after all, adding the kinds of experiments done there with D's, or Monkey D.'s ability to make trouble, was a recipe for disaster.

With that in mind, an idea formed in Issei's head. What if his father was in that lab? After all, it was probably the perfect place to hide from them.

(At that time, Luffy was in a completely different location, and was fast asleep.)

'He can only be here! Hehe, now I got you old man! But, how do I enter…'

Suddenly the door began to open, moving to its side, as an automatic door. Issei quickly hid in the corner of the hallway, trying not to make any noise.

"If I increase the power to a third... no, I think maybe the energy should be redistributed more times... maybe, like this... hmm, would a star work? I must get materials that serve this role in the floating sea world..."

Vegapunk, perhaps one of the greatest minds in the multiverse, left the room looking at a floating screen in front of him. As usual, he had his tongue hanging out of his mouth, and the top of his head was still a satellite, but now it stored much more information than in the past.

He was so focused that he didn't even notice Issei in the hallway, or the as boy entered in the lab. It was so easy it was surprising the kids didn't get into the lab more often. (They did, with ease. Most were able to keep it a secret, and they were almost never noticed.)

There were several devices scattered around the lab, which was gigantic. Issei didn't understand how something so big was there (he would later find out that this was the result of spatial manipulation, courtesy of one of Vegapunk's inventions.) He looked around, looking for signs of even a silhouette of his father, only to once more find nothing.

He wasn't satisfied with just that, and kept looking until around. The lab was not very organized or clean, so soon enough, the kid ended up stumbling upon something.

- Arg! Shit! What is..

His eyes were instinctively drawn to the object that caused him to fall. It was a fruit that was shaped like an apple, but it was blue instead of red and had its body covered in spirals.

- Devil fruit..

Obviously, he knew what the fruit in front of him was. He had seen them a few times, they were common in his family, mainly because of the Vegapunk experiments.

The boy stretched out his hands and lifted the fruit to the front of his face.

With the fruit in face to face with him, strangely his observation haki seemed more attentive than ever. He felt a mysterious strength coming from the fruit, and at the same time some sort of attraction to it.

Though he knew what a devil fruit was, he had never paid attention to how they were formed, after all it wasn't something that interested him. Even if he had seen some of his family eat fruits similar to the one in front of him before, he never thought much about it. Now it was different.

Now with the fruit in hand, he was made aware, even if they didn't explain it to him.

That wasn't a power of this world, it wasn't a power of any world. The fruit seemed to go further, it seemed to touch in even more primordial and basic concepts of reality. Issei couldn't explain it, and maybe he would never be able, but he somehow could feel his haki resonating with the fruit.

Instinctively, he took a bite.

...

"YOU LEFT THE FRUIT ON THE GROUND?"

Hancock's anger could be heard throughout the ship and further, along with her king's haki. And yet Vegapunk didn't seem affected.

"I did not consider that one of the children would enter the laboratory and eat it. My mistake, I admit, but I don't think this is necessarily something negative for the little one, is it?"

"It isn't something negative?"

It wasn't Hancock who answered, but Bonney, who was just as irritated as her sister-wife.

"Look at him Vegapunk!"

Bonney yelled, pointing at Issei.

The boy was on top of the dining table, but his appearance was vastly different. Parts of his body were covered in blue scales. His left arm, from his elbow to his hand, was no longer human, and for the older members of the crew, it resembled Kaido's arms when he transformed.

Issei's left eye had a reptilian appearance, with his pupil shaped vertically, and several scales covered the area around his eye. The teeth in his mouth were now sharp, and from the base of his back, a tail, that was almost as long as his leg, grew.

But, besides all of that, there was a big piece of meat in front of him, which he was devouring. The act of eating such a big piece of meat would be normal for the boy, if the meat wasn't raw. He devoured it savagely, resembling a beast more than a human.

"I think that's enough."

Even Luffy was serious at the moment. He extends his hand towards Issei, a layer of darkness covering his palm. The scales, tail and teeth are gone, and Issei's arm went back to normal. Chunks of raw meat were between his teeth, and the blood from the meat smeared his mouth. The boy was clearly disoriented and confused.

The reason why they had left him like that until now was because, inevitably, he would need to satisfy his urges, his hunger, and maintaining some rationality, the bestial side that the boy acquired because of the fruit would hardly be satisfied. Luckily, they had Luffy to completely nullify the fruit's effects when necessary. The instincts still needed to be satisfied, but for now Issei would be ok.

Luffy patted his son head.

"Vegapunk, this fruit is..."

Law inquired. His eyes were narrowed after seeing Issei's partial transformation.

"No, it's not another copy of the Seiryu's fruit. It is actually the Ryu Ryu no Mi, model Leviathan."

The crew had to hold their breath.

The reason why Issei acted like that was quite simple. Devil Fruits inevitably affected the user, especially Zoans, as they were fruits that practically had consciousness, fact attested by the various objects that came to life after Vegapunk inserted a zoan in them. Carnivorous Zoans in particular increased the user's instincts, were more brutal and violent, and had more power in how they affected their user. Above that, Issei's fruit was a Mystic Zoan that also came under the classification of Carnivore. Someone in teenagerhood or adulthood would already be heavily affected by the fruit, but a child? Instincts and savagery could easily overcome conscience, just like now.

Luffy went through something similar when he caught an Ancient Zoan that also entered the classification of carnivore, and Saul, had the exact same problem as Issei, since his fruit, the Mystic Zoan of the Fenrir, was of exact same type as Issei's.

At least they were not inexperienced in the matter.

"G-guys..."

And even so, seeing the confused and scared look on the little one's face, who didn't understand what was happening to him, they were hurt.

"Shhh, don't worry." Luffy whispers, stroking Issei's head, and giving him his typical smile that always calmed all of them down. "It's gonna be okay. We will help you."

"My baby..."

Hancock ignored Vegapunk and hugged Issei tightly.

The boy was scared, but he knew his family was there for him.

That was all that mattered.

...

"Don't fight it, it's part of you. Is not trying to hurt you or anything like that, resisting is counterproductive."

They were in an open field, with the Sunny floating meters above them. That wasn't the Straw Hats' original world, but it served to help Issei.

At the moment it was Saul who instructed him, as he had gone through the exact same experience as his younger brother.

"Counter productive?"

Issei, however, is confused by his brother's vocabulary.

"It means that you are doing something that will not give results."

Saul explains quickly. Issei wasn't stupid or anything like that, but he was a five-year-old who shared a lot of similarities with their father. Which meant that, academically, he was practically no good.

"Ah, I see..." Issei nodded, showing that he understood his brother's explanation, but soon his expression got worst. "But... what if I lose control?"

On his right wrist was a handcuff made out of seastone, a type of mineral that had the same properties of the ocean and worked to stop a devil fruit user from using their powers.

Issei himself held the key to the handcuff with his left hand, and now was looking hesitant at Saul.

"If you lose control, we'll find a way to make you take it back. Do as Saul say, resisting the fruit, which is now part of you, will only make you weak. And that's the opposite of what you want, isn't it little brother?"

Said Monkey D. Boa Paison, Hancock's first son, and Issei's older brother, who was of same age as Saul. The two even looked very similar, but Paison was taller and his dark hair was longer.

His brother's words were exactly what Issei needed to hear. Not a single one of the kids in the crew wanted to be weak, especially Luffy's sons and daughters. Without exception, all of them wanted to reach, perhaps even surpass, the heights that their father's crew members reached. And above that, they wanted to walk at the same height as Luffy himself had arrived. But that was hard, ridiculously so, considering how much of a monster their father was.

'The fruit I ate… I know it's strong… Argh! Come on, I want to do so much more!'

And, Issei in particular, had a lot of desire to be strong, arguably more than any of his siblings. He was still one of the youngest children, he hated the feeling of constantly being watched when they went out, or having to be protected.

Furthermore...

"Dad, is it nice to have so many wives?"

Issei and Luffy were taking a shower. To this day, the only way to get Luffy to bathe constantly was either to make him shower with one of his wives, or by making him bathe the children.

"Hmm, it depends. When I first met Sanji, he would probably love it, shishishi. But that's not how you have to think."

Luffy said, while shampooing Issei's hair.

"What do you mean?"

The boy looked confused.

"Well, I love your mothers." The father said with a big smile. "One day you will understand, be it one or several. Shishishi, honestly, it's a feeling I can't explain."

Issei then made an understanding expression.

"A mysterious feeling!"

"Exactly."

Luffy replied, still smiling.

"Hmm, now I want to understand this..."

The small one said thoughtfully.

"One day you will understand." Luffy then seemed to think of something. "Well, people keep saying that your brothers already look like me, so I don't think you'll be any different... your mother especially, will probably love having grandchildren..."

He already said the last one more to himself than for his son, but Issei still understood him.

"So, one day I'll be like daddy and also have several wives!"

That made Luffy burst out laughing.

"Shishishi, I know you will. Make sure to very strong to protect them."

The father said, stroking his son's head.

"I WANT TO BE STRONG TOO!"

The boy now seemed to have his motivation renewed. Without hesitating anymore, he used the key and took the seastone from his wrist.

"AAAAAAHHHHHHHHHH!"

Issei let out a scream, not of pain but of excitement. He did exactly as Saul suggested, not resisting or fighting his instincts, but instead embracing the strength of Leviathan's fruit. And without a doubt, it was effective. It was a rather strange feeling. It was like he was sinking at the same time as he could breathe better than ever. Suddenly, in his mind, large red serpent eyes looked at him, at the same time the pupils of his own eyes become a vertical line and the color of the pupils turned into the that same shade of red.

From both his mind and his lungs, a roar rises from deep within.

His brothers jumped back, as they would first observe and only later interfere. Those who would take care of Luffy at the moment were others.

"We're counting on you two."

Saul said, looking back at two creatures from the world where they were in.

One was a two meter tall monster with a body that resembled that of a dinosaur, but instead of scales, his body was covered with a thick metallic layer, almost looking like some kind of armor. Most of his body was gray, except for his head, which had two horns, and some other details, that were white.

The other was a much smaller monster, barely seven hundred centimeters tall, making a stark contrast with the gigantic metallic hammer the creature carried. The monster had hair that didn't really look like fur, but rather a fluffy mass. His body was mostly pink of different shades, with the exception of small parts that were of few other colours.

- AGGRON!

- Tinka!

The two yelled, already running to face Issei. Saul had a drop of sweat running down his forehead seeing the little Tinkaton carrying that gigantic hammer, while Paison laughed at the scene.

Gray clouds quickly formed in the sky, and a windstorm started in the area where the group was. These were just some of the effects of the powers of Leviathan's fruit.

Agron was a bit slow, so Tinkaton was the first to attack. He raised his gigantic hammer and brought it down on Issei's body. The hammer moved like a bulk, but it was possible to see a kind of pink aura covering it.

Issei's body is knocked to the ground because of the hit, and surprisingly, some damaged seems to have been done to his ridiculously tough scales.

Following Tinkaton, Aggron also punched the user of the Leviathan's fruit, while he was still on the ground. It was possible to notice that an aura came out of the monster's fist when he struck.

A roar from Issei throws both back a bit, giving the boy space to get back in the air. The wind intensified, arcs of electricity formed around Issei's body, and a blue light began to come out of his mouth, just as a blue aura began to cover his body.

Tinkaton and Aggron got ready, a pink aura with gray details covered the first, while a gray aura with brown details covered the second.

Aggron throws his head back before opening his mouth, causing a blast of cold air to come out of it, heading towards Issei. Tinkaton thrust his hammer forward, and a bolt of gray energy shot out towards Issei, who in turn responded to both attacks with a blast of blue fire from his mouth, as well as electricity that coursed around the fire.

"What are his chances?"

Paison asks his brother.

"Fifty fifty."

Saul answers, while writing some things in his notebook. It would be a long training session.

yes, I know how devil fruits works. That's not what happened to Momo, I know, I am doing my own thing, relax

some Pokemon introduced, they will be interested when we get to Kiyome, since that's the pretty much the obvious thing to do.

About those pokemon at the end fighting Issei, if you don't know them, Agron is earth and steel, and Tinkaton is fairy and steel. Steel is resistant against dragon type, and fairy has downright immunity to dragon type, that's why especially Tinkaton was hitting hard.

and yes, besides Ddraig also has a dragon type devil fruit. Leviathan is probably my favorite mythological creature, and it works perfectly for some stuff I have in the plans

about Issei's family, don't worry too much about it, they will have some presence on this earlier chapters, but pretty much none when we get to DxD. Also, keep in mind I am only showing cuts of time, so we are obviously not seeing all the crewmembers, or kids

Review:

Frank Winslow: Yup, they have a lot of simillarities, both in personality to some level and in their roles, or even some more coincidental ones. Either way, Issei being Luffy's kid is a crazy idea when you first think about it, but when is not so weird when you stop to look at the details