Disclaimer: I don't own Harry Potter or Kenichi the Mightiest Disciple, so the only people who are mine are the ones that don't appear in the Anime or Harry Potter.
After James's and Lily's deaths:
Sirius ran, he had to find Wormtail and teach him a lesson but then… he remembered. What about… what about the little girl? Kira Black, his child. He knew she was his, but he didn't know who the mother was. The morning before he awoke to a loud banging at the door. He went to the door and opened it. He didn't find anyone there. What he did find, however, was a small basket, a cot almost. There was a child, more of a baby really, lying there. There was a letter and after reading it he realised that the child in the basket wasn't any child, it was his, but… how? Where had it come from? He couldn't recall, and he'd completely forgotten everything that happened the year before, and it really was a year since the child would have been conceived. The letter informed him of who the girl was and of her birthday and some other things, like a special spell that had been placed over the child, so that in the case that he, too, was unable to look after her, she would be sent to someone who knew exactly who she was, but… he was doing this knowing that he would be leaving the child with someone who he didn't know. Her birthday was on the twenty-first of June, but if he went through with this, she wouldn't have even celebrated her first birthday with someone she was related too. Thinking about that made him hurt, but he couldn't let Peter get away with what he had done.
Friends stab you in the back… I thought there was no truth to that… it seems that I was wrong… Sirius thought bitterly. Peter had been his, Remus's and James's friend. If it hadn't been for him and James, he wouldn't be an Animagus, and if Remus hadn't been there, they'd never have been friends or seen any need to become Animagi. The least Peter could have done was repay them by not betraying them. And that wasn't the worst thing either. Everyone would believe that he, Sirius, the one who was willing enough to put them into Peter's hands for protection, would be blamed for betraying them. He would be sent to Azkaban. But, he as good as betrayed them, by persuading them to make Peter secret keeper. It was his fault that they were dead, and no matter how much he wished it were not true, it was mostly due to him that they were now lying dead in their ruined house and Harry was going to go and live with Muggles. He deserved Azkaban, but the child… his mind kept drifting back to the young girl. He hoped that wherever she was being sent, the people there would look after her…
He found Peter, cowering. "You should have known that if Voldemort didn't kill you, then I would!" He drew out his wand and was about to curse him when he was stopped by Peter. He laughed, laughed at the situation, at Sirius, at what he'd done. Sirius saw him take out his wand and hide it behind his back. A flash of light and Sirius saw Peter's finger, bloodied and severed from his hand, fall down and hit the pavement, saw a small rat disappearing into the sewage below and all the Muggles within the twenty foot radius drop dead, the curse Peter had used Sirius didn't know, most likely Dark Magic. He cursed. Ministry of Magic wizards came and saw what had happened, they asked the Muggles around that were alive what had happened and then they came up to Sirius and bound his hands. He felt a laugh bubbling in his stomach. Genius! He cut off his own finger and blasted a whole street apart as well as turning into an Animagus at the same time. It was no laughing matter, but he couldn't help himself, he found it funny, but not in the joking way, in the disgusted way. This wasn't the first time he had thought this, but he was ashamed to be considered a friend of Peter Pettigrew. They led him away, and threw him into a cell in Azkaban. The only thing that helped him keep his sanity was that he knew he was innocent, but that wasn't a happy thought, it was anything but a happy thought. The Dementors couldn't take it away from him and after a while he transformed himself into his Animagus form and eventually broke out twelve years later, to kill Peter. Well, we all know what happened after, and I'm not going to repeat it, all that needs to be said is that after that he lived out the rest of his life running from the Ministry of Magic.
Read and review. I know it's probably not the best, but it will improve over time, hopefully... But, I hope you like the Prologue! :)