A man or woman that a person keeps within a relationship for the sole purpose of not being alone; Company in bed at night and a sexual playmate. The bedwarmer doesn't always know it and is under the impression that the person actually loves him or her.
Ann: Why do you keep him if you hate everything about him?
Jane: I get lonely at night and I don't want to just sleep around with random men
Ann: So he's just like your bed warmer then?