What is an ambiguous word for a romantic partner, which doesn't make it crystal clear that they are in fact a romantic partner rather than just a friend? A euphemism might be a rough approximation of this, although of course it doesn't quite fit the bill, and some are not ambiguous at all.
Genderless and gendered answers are both interesting, though I'm personally interested in one which could apply to a girlfriend.
Ideally it'd be something which leaves the possibility that it's a romantic partner open, perhaps quite strongly so. 'Friend' suffers in this respect, as it's often used to make clear that someone is "just a friend", and people have suggested to me that girlfriends or boyfriends might be offended by it.
Anyone familiar with British culture or certain subsets thereof would understand why all this is vitally important ;)