A Quiet Life Seems Perfect

https://www.webnovel.com/book/the-demonic-mercenary-turned-babysitter_32578131008035605 (This novel is being rewritten in this book instead) Raito Yakamura followed strict rules in life. He believed in discipline, being efficient, and keeping control. He used to work as a mercenary but left that violent life behind for a peaceful civilian life. Everything changed, though, when he met Kurai, a clever and mysterious demon, and his world turned upside down. Kurai is not just a voice in his mind; she wakes up new powers inside him. She makes him stronger, quicker, and offers him abilities that are beyond human limits. But with this power comes a tough truth: he is no longer just Raito. (I will appreciate it if you readers leave some feedback in the reviews)