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This could be because they're normal roommates (ie college or post-graduation) or something more fantastical such as
- a being that fell from the sky and has taken up residence with a civilian (ie Asobi ni Ikuyo, Sora no Otoshimo, etc)
- a person being transported to an alternate reality where they begin to live with someone
- New recruits to a military organization who live with an established member (ie Shinji and Asuka from Evangelion)
Anything that involves two or more characters who barely know each other becoming roommates fits the bill.