The Evil Twin

Oh, not you again, me.

The twin or clone who is essentially someone else, likely the Hero, but evil. They likely fight against their good counterpart at some point as only one version of the character can exist. Obviously, as they're a twin of the original character, the Evil Twin shares identical attributes with them: same look, same voice (or a similar, more evil-sounding one), and same fighting techniques. When it comes down to figuring out which one is the good one, the Hero's friends cannot decide which one is the real deal because they lack the audience's powers to be able to look someone straight in they eye and know for a fact that's the twin and not the real person. If it wasn't made clear, the Evil Twin also has a high chance of being a Paper-Thin Clone.