When you go around fighting crime all over the galaxy the way the Legion does, you're bound to make some enemies. And given that you're, well a legion, the odds are that some of those enemies are going to form teams of their own.
Sarya of Venegar is a member of the Fatal Five, a group of some of the Legion's deadliest enemies, as the Emerald Empress:
The Empress controls the Emerald Eye of Ekron, a mysterious artifact of immense power that responds to her mental commands.
One of my favroite things about the Empress is the fact that, at one point - because he saved her life - she had the hots for Superboy. She even did the whole, "Let's run off together and with our combined power we can rule the universe!" thing.
Silly Sarya; don't you know that green is his least favorite color?
(Also, there's the whole thing about her being a vicious, power-mad sociopath. Then again, that never stopped him from being interested in Lana Lang...)