Maenads

The mysterious girl that looks like Rydia is an important antagonist of Final Fantasy IV: The After Years.

Storyline
It is revealed near the end that these girls were mass produced in the shape of Rydia by the true antagonist of the game in the True Moon and are known as the Maenad. these girls were sent to take control of the Eidolons, retrieve the crystals and destroy anyone who gets in their way.