Ironically, the first voice we hear in the Teen Titans cartoon isn't the Titans at all, or Slade, it's in fact the Headmistress of the H.A.E.Y.P (pronounced Hive) Academy. The headmistress is the one who would train the trio who would become some of the biggest thorns in the Titans side: Jinx, Gizmo, and Mammoth.
However, interestingly enough, she would disappear not long after, even getting a joke put on her via a "Missing" picture on a milk carton. Then, in the final season, she appears alongside The Brain's forces.
Though she's not the most known villainess, she is one that made an impression with the time she had.