coinhabitant; cohabitant; housemate
Refers to a person who lives together with someone else in the same dwelling, without implying a romantic relationship. Neutral term.
新しい同居者を探しています。
I'm looking for a new housemate.
彼は私の同居者です。
He is my coinhabitant.
Compound of 同居 (どうきょ, 'living together') + 者 (しゃ, 'person').