From Bulbapedia, the community-driven Pokémon encyclopedia.
Cassidy (Japanese: ヤマト Yamato) is one half of of the Team Rocket duo that occasionally runs across Ash Ketchum and friends in the Pokémon anime.
Cassidy's partner is Butch, and she is one of the few that usually manages to get his name right, however she has gotten his name wrong on many occasions- much to his frustration as everyone else also calls him either "Biff", "Bill", "Bob", "Hutch", "Butcher", "Patch", "Botch" or "Buffy", among others (コサンジ Kosanji in Japanese).
Cassidy and Jessie met either before or shortly after their both joining Team Rocket, and are assumed to have been rivals since then. The two women are almost equally vain. However, unlike Jessie, Cassidy keeps her cool easier and is much less prone to violent outbursts. Cassidy annoys Jessie to no end with her arrogance, and it appears that Cassidy likes instigating this frustration. She also often referenced to Jessie, her awareness of Giovanni's dislike towards the Trio, and that they were likely to be thrown out of Team Rocket, something that Jessie usually ignores.
Cassidy is named after Butch Cassidy, an American outlaw. Her Japanese name, Yamato, is named after the 大和 Yamato, a World War II-era Japanese battleship. Its sister ship was the 武蔵 Musashi, one of Jessie's namesakes.
In Grating Spaces, Delibird was delivering the Poké Ball containing Charizard to Cassidy, but it mistakenly gave it to Jessie. Delibird returned later and reclaimed the Poké Ball. It is assumed Charizard was delivered to Cassidy successfully.