I think friend or good friend fit the bill if ambiguous is what you are going for.
I spent the weekend away with a friend of mine.
I spent the weekend away with a good friend of mine.
Are they lovers? Or just friends? It's not clear; it kind of hints at that, but there's plausible deniability.
Less ambiguously, you have lady friend or guy friend.
I have a lady friend coming around for dinner tonight.
Lady friend especially, is a straight up euphemism for someone you have some kind of sexual/romantic relationship with.
