The Traitor

Top 10 Anim-- no. Just no.

The one who betrays their allies and joins the enemy side. The Traitor is likely one of the story's former protagonists to make things more dramatic. The Traitor can have their reasons. Such reasons can be that their allies suck, the other side is better, both, or they were coaxed into joining by somebody on the enemy side.

The Traitor's Mirrors
Nutshell: The one who switches from one side to another.

Enhanced: The one who temporarily switches sides to act as a spy on the other side for their allies. Shift: The Good Spy.

Cruel: The one who switches sides, temporarily or permanently, to get revenge on their allies for what they've done with the aid of their former enemies. Shift: The Bad Spy.

Inverse: The Loyal.

Atypical: This character could've been kidnapped by their enemies and forced to work for them using many methods (brainwashing, hypnotism, etc.).

Wrong: No Mirror.