katarina (fire emblem)
A playable character in Fire Emblem: New Mystery of the Emblem, Can form a good bond with Kris (My Unit) if the latter is male.
At first she sides with Marth, but it is revealed that she is actually Eine (Reese in English versions), an assassin serving Eremiya attempting to kill Marth. Once discovered, she serves as an enemy and later ally if re-recruited by Kris.