Nega Starfire is an evil clone of Starfire created by Trigon.
History
Since Cyborg, Starfire and Beast Boy are protected to Trigon's powers thanks to the Ring of Azar, he refuses to expend more of his energy to destroy them and uses his powers to create evil clones out of each of them and pits them against each other, each against his or her own clone.
During his battle, Starfire is evenly matched with Nega Starfire. Nega Starfire taunts her by saying that when she wins Robin will be hers.
Later, as the tide of the battle above ground starts to turn against the Teen Titans, they decide to switch combatants in hopes of regaining the advantage. Therefore, Beast Boy fights against Nega Starfire and manages to beat her by turning into an Apatosaurus.