Baoh Manga

From Wikipedia: An innocent young man, Ikuroo, has a parasite known as Baoh implanted in his brain by an evil organization, Doress. The parasite makes him nearly immortal and gives him the ability to transform into a really powerful beastie when he's in trouble. Doress intends to use him in some sort of ploy for world domination. But while they're moving him around, a young psychic girl, Sumire (who's also being held by the organization), sets him free, and the two escape together. Of course, Dr. Kasuminome is the mad scientist behind the whole thing, and he isn't about to let his test subject get away... and he has everything from superpowered lackeys to a small army at his disposal.