Denji got stuck with his father's massive Yakuza debt as a kid—basically inheriting a nightmare. He teamed up with Pochita, a small devil, and started hunting devils for the Yakuza to try paying it off. That didn't end well. The Yakuza turned on him and killed him, but Pochita wasn't about to let that happen. The devil merged with Denji's body, becoming his heart and bringing him back to life.
Now Denji can transform into a chainsaw man hybrid by pulling a cord in his chest. When he does, his head becomes a full chainsaw and he sprouts chainsaw blades from his wrists—basically turning into a weapon. After crossing paths with Makima, he ditched the Yakuza life and joined Public Safety as an official Devil Hunter instead.
Content compiled by AnimeList.moe from publicly available sources.
