Who was Headmistress of Beacon versing in Episode 1: Ruby Red Rose?
It is revealed that the woman was Cinder:




  Cinder Fall is a character in the world of RWBY. She appears to be affiliated with Roman Torchwick in some way, perhaps controlling, or at least coercing, Roman rather than cooperating with him


She is definitely a main character, she is also the antagonist and was the exact same woman that the headmistress was fighting against.