In the Teen Titans comic, one of the most important stories ever was The Judas Contract, in which a girl named Terra joined the Titans, then betrayed them. So, naturally, when the cartoon series arrived, it was only a matter of time before Terra appeared, and sure enough, in Season 2, she did.
Now, they played this one a bit more tv-trope wise, she was a girl who couldn't control her powers, almost joined the Titans, then was convinced by Slade that he could help her more than they could. She became evil, and helped Slade take over Jump City, and almost kill the Titans.
Then, when she came to her senses, Slade used technology to control her, and it took all her willpower to not only fight back, but to take down Slade forever. At the cost of her life though.
She did "return", but for many Terra died beneath the earth...a hero.