R3-S6 was an astromech droid in the service of the Republic. He was subverted by CIS intelligence forces and reprogrammed as a spy for General Grievous. He was then assigned to Anakin Skywalker as a replacement for R2-D2, who had gone missing during a recent mission.
During a raid on a CIS listening post over Ruusan, Anakin Skywalker, Ahsoka Tano and Captain Rex were taken in by R3-S6's betrayal, which led to various setbacks. R2-D2 resurfaced during the mission, and defeated R3-S6 in combat.