Chainsaw Man, Vol. 1 by Tatsuki Fujimoto

Denji’s a poor young man who’ll do anything for money, even hunting down devils with his pet devil Pochita. He’s a simple man with simple dreams, drowning under a mountain of debt. But his sad life gets turned upside down one day when he’s betrayed by someone he trusts. Now with the power of a devil inside him, Denji’s become a whole new man—Chainsaw Man!