Yui Serimochi

Yui Serimochi appears to be one of Juri’s college classmates. In reality, she - her true name being Arche - is one of the goddesses of the Rainbow World, along with her sister, Iris, but she was exiled for evil behavior. She was the one to give Shana her powers at the beginning of season two and eventually goes on to become the main antagonist of season three.

Trivia

 * Yui's real name bears a close resemblance to that of Arke from Greek mythology.