Evil Emmy is an evil ego of Emmy.
Evil Emmy is a 6-year old ego of the good Emmy, only she has red eyes.
She and Cynthia have a good connection. Every time Max and his dragon friends try to get Emmy back, Cynthia orders Evil Emmy to back off.
In Emmy and Max's Adventures of Scooby Doo and the Reluctant Werewolf, Cynthia has a talk with Emmy, and wishes Emmy to stay with her forever. Then she kisses Emmy on the lips, and her eyes turn red making her evil.
In the end Max confronts Evil Emmy and tells her that this is not the Emmy he knows. He tells her all the good times they've had together and adventures they've been. Then he tells her he loves her. Hearing this, Emmy's eyes turn brown, and has turned good.