Boku to Majo no Jikan

What should be the luckiest day of his life ends with the boy dead—or so he thinks. He wakes in a laboratory beside a strange girl who says she wants to examine him. His appearance has changed: his hair is now white and his eyes red. He also discovers that nothing seems able to wound him. His apparent death and unexplained transformation leave him facing an unfamiliar situation with the girl who is studying him.