Could you be thinking of Shinichi Izumi of Parayste?
Shinichi is the protagonist of the manga, a thoughtful, compassionate high-school boy whose hand is infected with a Parasite and is repeatedly put into difficult positions. He must find a way to peacefully coexist with Migi, the Parasite which has taken over his hand, and reconcile his desire to protect humanity from the Parasites with his desire to keep his own Parasite a secret in order to avoid being killed or used as a laboratory specimen. Like a superhero with a secret identity, he must also find a way to explain away his Parasite-fighting activities, as well as the stress and grief they cause him, to his friends and family. While originally forced to have Migi fight for him, Shinichi later gains heightened abilities when trace cells of the Parasite course through his body, and fights his own battles, with the two having an advantage in both being able to act independently and work as a team. Shinichi's retention of his humanity, despite gradually becoming emotionally distant as a side-effect of Migi's cells, makes most of the other Parasites deem him a threat. After defeating Gotou, with Migi deciding to "go to sleep" indefinitely afterward, Shinichi attempts to live a normal life again while having an understanding of natural order from his experience.
Migi tends to take over in times of danger, which often manifests itself as blades.