Someone of the opposite sex, male or female, often an intimate friend, with whom one engages in make-out type activity (often while watching a movie), or even actual sex, but where said activity is limited to the couch and is not reflective of the overall relationship.
Sue: "Mary, is that your boyfriend over there?"
Mary: "Naw, he's just a Couch Companion..."
A hot but down to earth girl who makes a great 'couch companion', someone both fine and fun, equally able to be insanely sexy or low key and comfortable, usually at the same time.
Check out Ana in that fluffy sweater...she is totally sextable.