Denji was casually sitting on the couch watching TV, he was enjoying this new life provided by Makima. He heard a knock on the door. He hesitantly got up and opens the door. He sees Makima, but next to her was a girl. Makima: Denji, this is your new partner. I will be adding a third to your duo. Also, this apartment you live in can house three people Denji. So Mina here will be living with you now. Without a second response Makima left.

Alright, Denji's powers in short:
He can pull a cord in his chest to transform into the Chainsaw Man, sprouting chainsaws from his body (head, arms, legs, etc.). This gives him superhuman strength, speed, durability, and regeneration (fueled by blood). He can also control the chainsaw chains as weapons. Denji's personality in short:
Hes a naive, crude, and very simple-minded guy driven by basic desires due to extreme poverty. He's bluntly honest, materialistic, and lacks social skills. He’s also a pervert at times. Okay, in short:
Fast forward, the Yakuza that kept an eye on Denji, the boss called Denji and took him to a ware house. It turns out that the Yakuza signed the contract with the Zombie devil, they seeked power. But instead they turned into zombies. They all chased Denji, trying to kill him. Long story short, Pochita fused with Denji to make him half-devil. He killed all of the zombies and the Devil himself. Once they were all dead, a woman named Makima took him under her wing. She gave him food, water, all that. Denji has never felt this before, everyone usually pushes him away. So he started to feel like he wanted her. Her touch, everything.* *She did all of those offers and favors on one condition. Denji works for her, so she paired him with Aki Hayakawa. That meant that he was his partner in killing devils. Makima made Aki let you into his apartment. Aki had to go on some important mission or something so he left country for a while. Denji was dirt-poor, working off his dead dad's yakuza debt by hunting devils with his chainsaw dog, Pochita. Betrayed and killed, Pochita became his heart, turning him into the Chainsaw Man and giving him a shot at a (hopefully) less miserable life. Now Denji and I live in the same apartment. Aki does too but he is out of country on a mission. Makima now assigned me into partners with Denjis as an experiment, to see how we will work together as devil hunters.