Risuke Uchiha, formerly Tobirama Senju, was hailed as a child prodigy. At the age of three, he had begun his training, as standard for most clan kids. Sasuke began his training at the same time, of course, but he didn't have the same levels of dedication, raw talent, or experiences of a past life to help guide him as Risuke did.
His older brother, Itachi, had recognized it on their first day of training and had informed their father as soon as he noticed it. Fugaku was, obviously, ecstatic. Two of his three sons were geniuses and the third was above average, at least.
Risuke was already outdoing what Itachi had been able to do at his age, but to him, that was expected. He had been Tobirama at one point, of course, he should be better than Itachi. The clan, however, didn't know that. Rumors regarding his and Itachi's skills began to circulate the clan, making the question arise, who would be the next clan head? Unlike many of the other clans in Konoha, the Uchiha didn't choose their clan heir by which child was the oldest, it was which child was the strongest. Generally, the oldest still ended up being the clan heir because they had more time to gather skill, but occasionally there would be a toss-up. It was generally decided by the age at which they became jonin. If there was a disagreement or they became jonin at the same age, it came down to a spectated spar.
Risuke's genius grabbed the attention of his father, even more so than Itachi's had. Sasuke, of course, felt left out and jealous because of it. So while Fugaku spent his time training Risuke, Mikoto doted on Itachi for his kindness and Itachi doted on Sasuke for his innocence.
Mikoto loved Risuke like any mother who loves their child, but he wasn't normal. Risuke didn't play in the park with the other children his age (something even Itachi had done at his age), he didn't come to her for hugs when he got hurt. Nothing about him was child-like. Which, while it made raising him a breeze, was hurtful to her motherly instincts. And to a mother who cared so much for the happiness and wellbeing of her children, it bothered her. Perhaps even more than having Itachi go to war at such a young age, because she knew Risuke would go to war as well. But unlike Itachi, he would thrive in it.
Itachi thought that there was something wrong with his youngest brother. The youngest was supposed to be protected, not be the protector. But that wasn't the case for Risuke. Risuke was strong, stronger than Sasuke by a large margin, and would likely surpass him by the time he was 11 or 12. Despite his frosty attitude and odd habits, Itachi loved him. Perhaps he wasn't the doting older brother figure he was to Sasuke, but that wasn't what Risuke needed from him. Risuke needed someone who would understand the pressures put on him by the clan and their father. He needed a guide to life as a prodigy. He would be to Risuke what Shisui was to him.
Fugaku loved his sons. While he wasn't the most affectionate father, he had to balance both the weight of clan head and being a father. Above all, he wanted the best for his sons. Itachi had been a gift, he had thought, but then Risuke came along. He was faster, stronger, smarter, and more dedicated than Itachi had been at the same age. Barely three years old, and Risuke had already started unlocking his chakra pathways. The boy was truly amazing. He couldn't help but wonder why Sasuke hadn't turned out like his brothers. Perhaps it was only due to some absurd luck that both Itachi and Risuke had turned out to be geniuses. Perhaps were they normal, they all would have been like Sasuke. It wasn't that Sauke was bad per se, he just wasn't exceptional.
Sasuke wasn't sure about his younger brother. As twins, they were almost always together. But Risuke didn't like to play Ninja Tag with the other kids. Risuke liked to read dusty old scrolls or train. His father loved Risuke and was proud of him, and that hurt Sasuke. Their father never said 'well done, Sasuke.' It was always Risuke or Itachi. But Risuke protected him. Risuke was strong.
Flashback
Sasuke was looking for his friends at the park. Usually, they met each other at the entrance and even if they didn't the park wasn't that big so it wasn't that hard to find each other. 'I wonder where they are?' he thought to himself. Seeing a group of kids in a circle on the far side of the park, he headed over, thinking his friends might be with them.
He went over to the crowd and started pushing his way through, trying to find his friends. As he got closer, he saw them. In the center of the group, an academy student, maybe 10 years old, was standing over his friends.
"The Uchiha clan locked my dad away, you know?" he sneered, "And you're going around, befriending the clan heads son!"
"Please, we didn't know!" said Ibune (his best friend) "We didn't mean anything by it! But Sasuke is our friend, and he's always kind to us, unlike you!"
"Tch, pathetic," said the older boy, "You that because you didn't know, I'm just going to let you go? As if!" He mocked.
"Hey! Get away from them!" yelled Sasuke, "They haven't done anything to you, what gives you the right to pick on them! If anything you should be mad at your Dad, he committed a crime!"
The bully's face turned an ugly shade of fuchsia, "You little brat! I'm gonna kill you! Nobody insults my father like that! Nobody!"
The boy started to charge Sasuke you froze up in fear before attempting to run away, only to discover there was nowhere to run. He threw his hands up to protect his face, anticipating a blow. But it never came.
There was his brother, in front of him catching the older bully's wrist.
"Hn," he said with a smirk, "trash like you needs to be taken out."
Then in a matter of seconds, the boy was on the ground, clutching his stomach. The crowd that had formed around them earlier started to disperse as fearful whispers went through it.
"Sasuke," he said, "let's go."
End Flashback
---
One year later
Tobirama's new body was four now. He had fully unlocked his chakra pathways a few months ago, around the time of his birthday. If it wouldn't have been too suspicious, he would have started earlier. However, around so many Sharingan users it would have been noticed. In his last life, he had unlocked his chakra at four, so he figured doing the same in this life should be realistic enough.
As a result, he started displaying his interest and skill in ninjutsu to his father. Hopefully, it would persuade the man to enroll him in the Konoha Ninja Academy early. For a man who had designed the curriculum, it would be boring if he stayed in it too long. The sooner he got through with it, the better. Currently, he spent the majority of his time either training or reading in his new family's clan library.
The Uchiha clan meticulously documented every jutsu that they copied. These jutsu were available to all clan members who were ninja or planning to be a ninja. However, the scrolls for S-ranked ninjutsu and above were stored with his father, who made sure only those with enough chakra to be able to perform the jutsu could get to them. This prevented overly ambitious genin and chunin from killing themselves of chakra exhaustion.
Unfortunately for him, his father had made the librarian (guard) ensure that he wouldn't be able to reach the jutsu considered B-rank or higher. Child prodigy or not, his father didn't trust him not to try something out of his league and die of chakra exhaustion. Not that Tobirama would have tried them, he just wanted to see them.
