Unwilling Antagonist

The character is an antagonist for the main characters. The key difference is that they oppose them against their own will.

There are basically two main reasons for why. They might be coerced/blackmailed by another character to do that, or they have been brainwashed/mind controlled.

Frequently the trait will be a spoiler, as the real reasons why the antagonist opposes them might not be immediately apparent to the main characters.