Word for neutral antagonist/opponent

I'm looking for a word or two that describes someone that a protagonist interacts with, but is not necessarily a rival or an ally, but could be. That entity could also be neutral, or change from one to the other.

Things I've scrapped:

• antagonist: they don't have to antagonize the protagonist
• opponent, adversary, contender: they're not necessarily competing in anything
• conversational partner: they're not necessarily friendly and might attack, or might not even speak to the protagonist

What I'm considering right now:

• encounter/encounted: seems most neutral, but etymological it means meeting an adversary (according to google dictionary)

sample sentences:

• Your _______ stares at you belligerently.
• Your _______ smiles at you.

Solution 1:

There's a novel by Robertson Davies called Fifth Business, which describes a character somewhat along the lines you describe, named Dunstan Ramsay.

According to the Wikipedia article, Davies later confessed to having made up the term himself. The novel is no longer so widely-read, so I'm not suggesting that Fifth Business is right for your purpose. However, a lot has been written about Davies and his writing, and in one of the more academic works you might find exactly the word you want.

A search for "Dunstan Ramsay" on Google returns a large number of hits, including a WordPress blog devoted entirely to explaining the book's main characters in terms of Jungian archetypes!

Somewhere in this I found the term character foil, which might suit your needs.

Solution 2:

Working under the assumption that the person who may become your antagonist is not a good friend of yours, I think acquaintance fits best there. Otherwise, contact would also work.

Edit: By chance, I happened on the word interlocuter today, which reminded me of this post. It's a little formal, but it matches your request perfectly.