Angel and Demon (Mavis Vermillion x Zera "Big" Brother Reader Demon King)

(May make this Harem maybe, but for now, only Mavis (Zervis will happen, but she will end with you due to plot armor))
You are Zera's younger brother, but since you have great magic that rivals all kinds of magic, everyone respects him, even his old man. He was the only one who was nice to Mavis and threatened anyone mean to her, but since you took a lot of jobs.