Fake Fiancé, Real Grandmaster

Ian Ridge wanted to be left alone in the mountains. He reached the Innate Realm younger than anyone before him and had no particular use for what came after that, and then his master sent him down with a job he could not turn down, looking after the daughter of an old friend. The daughter is a researcher who has developed Panacea, a drug that slows aging and keeps people young, which is the reason she needs looking after. Ian arrives in the city where she works and, through a mix-up on his first night there, ends up in bed with her. In the morning she decides to keep the situation rather than undo it. Her family is pushing an arranged marriage on her and a boyfriend already in place is the quickest way to refuse it, so Ian stays on as the boyfriend in public and goes everywhere she goes.