(someone's) evil twin

Translations
злой близнец (кого-либо)
Whenever something went wrong, she blamed her evil twin.
Когда что-то шло не так, она винила своего злого близнеца.
Definitions
A humorous or fictional term referring to a person who looks exactly like someone else but behaves in an opposite, usually wicked or mischievous, manner. It is often used to explain uncharacteristic bad behavior or as a playful way to suggest that someone is not acting like themselves.