Chainsaw Man: Mangekyo Sharingan

Kotone Asaba is transported into the world of Chainsaw Man and awakens the Mangekyo Sharingan’s Kamui, in both eyes. He also receives an Affinity Codex, a system that grants rewards whenever he unlocks character profiles and raises their affection. Kotone protects key characters, pulls figures like Kobeni, Reze, and Makima into his orbit, and gradually builds his own faction while escalating into bigger conflicts involving later major Devils. *It's not the same fanfic as the other one with a similar beginning, that one has a different story and seems to be abandoned. — Description from novelcodex.org