I'm not sure how much backstory is enough but about 6 months ago a death in my wife's family caused my BIL (mid 20's) to sort of be stranded house wise. He lived with their mother since graduating college and I while he could have taken over the lease, I completely understand not wanting to live in a place your mother lived before she passed.
I wasn't hot on the idea even then but I didn't feel I had any say given the immediacy of it and also no plan on his end. My wife and I talked about it and we both conveyed this was temporary to him because we're looking to move early 2027 if not the end of this year. Something she doesn't know is soon after the passing he came to me asking if we all move together was I thinking of all of us living together again, and in so many words my answer was no.
Fast forward 6 months there's things that irk me. What used to be my wife and I office is now his room. I work from home so now my desk is in the living room (apartment is only but so big) He's more of a night person, and I'm a very early morning person 4am I'm getting ready for the gym he's up well into Midnight. We've had to talk to him about cooking at 10pm. There'll be times I get up in the morning on many occasions and the front door is left unlocked. He's got things delivered to the apartment, he isn't the cleanest (not filthy by any means) he claims certain things that he shouldn't. In short he's way too comfortable for someone living in another man's house in my opinion. But let's say none of the things that bother me happened. The sheer fact he lives here IS the problem for me. I don't want to ramble too much but wanted to get an idea or even help.