Betrayal

This character is one of the good guys, but also one of the bad.  She's gifted and deadly and uses her exceptional powers to get revenge on those who betrayed her.

Sarah Kerrigan

Before Kerrigan was the Queen of Blades wreaking her Zerg havoc across the galaxy, she was a ghost--an elite soldier trained in stealth.  But Kerrigan was also unique, she possessed powerful psionic powers.  Because of her skills she became a powerful tool of the Sons of Korhal.  She gets sent in to carry out a secretive mission and meets up with Jim Raynor.  Initially, Kerrigan and Raynor don't really seem to get along, but over the course of several missions their relationship improves.  Part of the reason why this happens is because of their mutual distrust of the leader of the Sons of Korhal, Arcturus Mengsk.  Mengsk is willing to do whatever it takes to put down the rebellion--including unleashing the Zerg on his fellow Terrans through the use of powerful Psi-emitters.  Kerrigan strongly disagrees with this tactic and urges Mengsk to stop using it.  Eventually Kerrigan and Raynor end up on Tarsonis where Mengsk, despite his promise to stop using the Zerg, has planted more psi-emitters to lure the Zerg to attack the Confederate bases.  As the Zerg overwhelm the colonies defenses, Kerrigan calls to be withdrawn only to discover than Mengsk has abandoned her.  In another amazing Blizzard cinematic from Starcraft II, players are given a glimpse into Kerrigan's betrayal and last moments as a wave of Zerg overtake her.

Players of the Starcraft series know that this isn't the end of Kerrigan.  The Zerg capture her and turn her into a powerful psionic weapon.  She becomes the Queen of Blades, a force of unstoppable psionic power, that is hellbent on seeking revenge on Arcturus Mengsk for his betrayal.  Throughout Brood Wars and Starcraft II:  Wings of Liberty, Kerrigan and her Zerg are formidable opponents.  All traces of the ghost Sarah Kerrigan seem to be gone and replaced by the Queen of Blades.  In the recent second episode of Starcraft II, Heart of the Swarm, players get the chance to play as Kerrigan and command the Zerg swarm.

I've always liked Kerrigan's character.  She has an interesting backstory and her quest for vengeance is engrossing.