Kurozakuro(2004)

Mikito Sakurai is tired of being a punching bag for all the delinquents on campus. But what can he do? By nature he's a gentle and easygoing high school student. That all changes the night he swallows a mysterious orb and meets Zakuro, a strange kid who promises to grant his most heartfelt desire. "When you wake up," says the pint-sized apparition, "you'll be stronger and better than a human."

And that's exactly what happens. Mikito wakes up with superstrength. But there's a catch. He may look the same on the outside, but deep inside lurks a monster that craves ultraviolence and the taste of human flesh. "I used to hate the sight of blood," says Mikito to himself, "but now it looks so beautiful to me."

(Source: VIZ Media)