I've read quite a few crossovers where Harry becomes or is related to different Katekyo Hitman Reborn! Characters through various means. These are my ideas on who he could become and how it could happen. I might develop some of these into more detailed oneshots or short stories later, but for now they're only rough ideas. If you want to use one, please just let me know! I'd love to see what you can do with them.
Chapter One - Reborn
Harry hesitated, potion in hand. This little vial was the last stage in creating his new life. He had the paperwork sorted. The life of Renato Sinclair had been meticulously crafted. Orphaned in a car crash as a baby, ran away from his abusive Aunt and Uncle who later died in mysterious circumstances. Various arrests for petty crimes, suspicions of much worse deeds that couldn't be pinned on him. All the details of Renato's life had then been buried. Anyone looking for his backstory would have to dig hard for it, and so they would assume that was all there was.
Harry was tired of this life. During the war, after Ron and Hermione had left the tent because of that horrific argument, something had frozen within him. He'd always known that he was capable of awful things. Quirrell's death had never bothered him, even as he remembered his screams, the feel of him crumbling under his hands. He had held this part of himself in check, fearing the reactions of his friends. He had crafted himself into the person they wanted, a person they would like. He wanted so desperately for them to accept him, but clearly that had failed. There was no benefit in being nice anymore, so he was going to take the fight to the Death Eaters.
He became feared. He was calm and patient, taking out his enemies when they were out shopping, exposed in daylight. A Sectumsempra to the neck, they were down, he was away. After they stopped coming out in public, he began to break into their homes. By May, Voldemort was down to his richest supporters, those who could cower behind protections Harry didn't dare to try and break. He attacked Hogwarts to draw him out, and Harry obliged.
Now Voldemort was dead, and Britain was safe. Ron and Hermione had apologised, tried to explain why they had left, how they had tried to return. They gripped each other's hands as they stared at him with tears in their eyes, but Harry was shocked to find that his burning need to please them had gone. He just didn't care anymore.
He didn't want to be their poster boy. He had no place in this bright, gleaming world that they were trying to build. So he planned to vanish. Muggles always had monsters walking among them, needing to be eliminated. Killing was what he was good at, and there was nothing holding him back now except him own code. So he created his muggle identity, using memory charms and forging paperwork. Renato was a work of art. He was quiet, arrogant, confident and debonair. He would also never go by his real name.
Harry downed the contents of the vial. His poor eyesight, the last thing he needed to fix. He stared in the mirror as his bright green eyes – his mother's eyes – turned black. He grinned to himself.
"Reborn," he said. "I'm Reborn."