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Some of the ideas you propose are intriguing, for now I'm thinking of the supernatural being / roommate one but I'd be willing to try other ones too. Here's some thoughts, let me know what you think:
The creature (C) could be a good old fashioned ghost haunting the place when the other person (RM) moves in. C tries to spook RM in a really tired fashion but RM is thoroughly unimpressed by its efforts - the inside of their head is scarier than this ghastly has-been. C keeps trying but eventually grows tired of its fruitless attempts to scare the living daylight out of RM. Might as well be friends.
RM summons C as part of some weird occult ritual they didn't even think would actually work. Now C refuses to go away and wants to help RM achieve some grandiose goal which RM doesn't give a damn about.
RM is just living peacefully at their place. C has messed up spectacularly in its supernatural career and needs a place to lay low. RM's house will do.