Cait Sith was a talking black cat perched atop a giant, white moogle. It was actually a robot controlled from elsewhere by Reeve, a top employee of ShinRa. Inc. While originally a spy who gave Tseng the Key to the Temple of the Ancients, Cait Sith continued traveling with AVALANCHE and eventually turned to their side, redeeming himself by sacrificing his puppet body to gain the Black Materia.
While Reeve continued to rebuild Midgar by forming the World Regeneration Organization, he sent Cait Sith (sans the white moogle) to assist AVALANCHE in their battle against Bahamut Sin in Edge. He fought alongside Red XIII, perched on his back.