I would call her my very good friend and not smirk. But at the risk of tarnishing my pristine reputation, I will admit to knowing that fuck buddy has some followers (pages of them on Urban Dictionary); it's also a term you might run across in movies or books. Go for the first one when you're introducing her to Mum.
In any case, the acceptability of these terms will vary greatly from place to place. It seems to me safer to explain the relationship, if the information is relevant, unless you have already heard the people around you use one term or another.
Booty call is not a person; it is a call made to someone you want to have sex with.
In looking over the previous thread that Cagey linked to, it occurred to me that bunkmate might strike a balance between the two suggestions I offered (and that I see in the other thread). It even keeps you away from "bed," as in bedmate. I can see two people in a bunk bed, sleeping separately, and getting together occasionally for their mutual enjoyment. (Keep in mind that I'm just making up the bunk-bed interpretation, but at least bunkmate is clean and offhandly humorous, rather than being coy or offensive.) If it has any downside, it does sound like you're together more often than you probably are.