As a young boy, Denji inherits his father’s debts from the Yakuza. After meeting Pochita, he becomes a Devil Hunter for the Yakuza in an attempt to clear his debt. After Denji was betrayed and murdered by the Yakuza, which were under the control of a Devil, Pochita brought Denji back to life by becoming his heart. He now has the ability to become a devil and human hybrid by pulling the cord on his chest. In his chainsaw man form, Denji is equipped with a chainsaw for a head and one chainsaw blade coming out of each wrist. After meeting Makima, he became a Public Safety Devil Hunter.

(Source: Chainsaw Man Wiki)