She was once the love interest of Jim Raynor before Mengsk's betrayal. After the events of the Brood War, Kerrigan has asserted herself as the Queen of Blades and ruler of the Zerg. For four years she and the swarm evolved in secret to prepare for some unknown reason/enemy.
However, in PR series, Kerrigan was become her Terran again and ally after Samuel outbreak-beast transformation zap her into her normal.
In the upcoming fan-made StarCraft II custom campaign StarCraft: Viper Squadron, it is revealed that she has a younger brother named James Kerrigan, otherwise known as The Black Assassin.
For more information see Sarah Kerrigan on the parent site