Dark Sai

Background
Dark Sai is a dark clone of Saithra that was created when Saithra thought of malicious thoughts before the Beginning Saga.

Jessica similarly to Saithra trains with Dark Sai in the Dark World (Realm), and as a result of this training Dark JC was formed from Jessica's inner evil.

Dark Sai isn't necessarily a villain per say, but rather he's more like an anti-hero.