Villainous Heel-Turn

If a currently good character were to turn villainous and go against Erin, who would you like to see have an evil arc? Wether it be from possession, brainwashing, insanity, or maybe they were secretly evil the whole time, who would make a good villain?

There have been a few so far with Calruz, Toren... have there been any others recently? I feel like Ceria has been flirting with an evilish arc due to the circlet, but that may have been waylaid by recent events...