Trope Finder

I don't know, is this a concept you just think would be cool, or do you have examples?

It's a case of video game tropes tending to be player-facing. A Unique Enemy stands out because you're mowing down hordes of otherwise identical enemies. It's odd for an enemy that isn't mechanically different, or a boss or miniboss, to receive a completely unique non-Palette Swap appearance. The opposite of this would be more like Unique NPC, but that's more the default: in games with a lot of interactive NPCs, they're more likely to have unique appearances anyway, so that they can add flavor to the surrounding environment, as well as so that you can easily tell generic NPCs apart from the actual Quest Givers, merchants/Intrepid Merchants, exposition fairies, and so on.

Well, someone added the Bloody Marine from Halo 2 to the Unique Enemy page, which is what gave me the idea(they even called him a Unique Ally). The trope would be just like Unique Enemy; the criteria for a Unique Ally would be a certain allied NPC(s) that is found once in the game, an ally with a unique skin(like the aforementioned Halo 2 Bloody Marine, but this can also be an ally who is found once), or a single ally(or group of allies) carrying a certain weapon that they normally don't carry anywhere else. Just like minibosses or antagonists(who are not necessarily bosses) that are fought as NPCs, friendly NPCs that are main characters(take Half-Life 2's Father Grigori for example, who carries an unobtainable weapon that only he uses) wouldn't count. This also doesn't include civilians or otherwise neutral NPCs.

My point is, we don't come up with tropes based on if we think they're interesting concepts. We come up with them if the pattern verifiably exists across multiple works. If you can demonstrate this exists, take it to Trope Launch Pad.