The first villains that the Teen Titans faced were a trio of teenagers from the Hive academy. They were headed up by a pink-haired girl called Jinx. In some ways, she was the most dangerous of the lot.
Jinx is technically defined as having "probability magic", meaning that she can affect how likely things are to happen. This makes her very dangerous in battle because she can make things happen to her foes that they can't fully prepare for.
In the show, Jinx had some interesting arcs. Including having a crush on Cyborg (when he was undercover for Hive) and even falling for Kid Flash (more on him later) and becoming a hero in the final battle against Brain and his forces. Just goes to show, just because you start out evil doesn't mean you have to stay that way.