Anime onde um menino se transformou em uma arma ao sentir perigo

É D.Gray-man

Graças às sugestões de todos, procurei Devorador de Almas no MyAnimeList e através das sugestões, encontrei o que estava procurando.

Losing a loved one is so painful that one may sometimes wish to be able to resurrect them—a weakness that the enigmatic Millennium Earl exploits. To make his mechanical weapons known as "Akuma," he uses the souls of the dead that are called back. Once a soul is placed in an Akuma, it is trapped forever, and the only way to save them is to exorcise them from their vessel using the Anti-Akuma weapon, "Innocence."

After spending three years as the disciple of General Cross, Allen Walker is sent to the Black Order—an organization comprised of those willing to fight Akuma and the Millennium Earl—to become an official Exorcist. With an arm as his Innocence and a cursed eye that can see the suffering souls within an Akuma, it's up to Allen and his fellow Exorcists to stop the Millennium Earl's ultimate plot: one that can lead to the destruction of the world.

Você poderia estar pensando em 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 tende a assumir o controle em tempos de perigo, que geralmente se manifestam como lâminas.