- "NO! We just get high together! Goddamn, you are so judgemental! Like you're a damn saint or something!"
- ―Louise Cassidy-Williams
As implied from her family name, Louise is the sister of Phil and Mary-Jo Cassidy, the wife of Marty Jay Williams, the leader of a local gang (as mentioned by Phil, by way of Vic's pager), the mother of an infant Mary-Beth Williams, and Vic's love interest. Early in the game, Louise resides in the a trailer park with Marty, but grows increasingly unhappy of her rocky relationship with Marty, eventually leaving his home, taking Mary-Beth along with her, and staying in Mary-Jo's apartment in Little Havana. Marty, angered by his wife's act of hostility, attempts to kidnap her but is ultimately been killed by Victor. After Marty's death, Louise acts as a business associate and Vic's love interest during the game, although their relationship turns sour later on.
Louise is later kidnapped by Jerry Martinez; his men take her to a lobby where Victor finds her savagely beaten. Louise has been killed by Armando Mendez after Armando's men kidnap her and took her to the Mendez Brothers' mansion. Armando shoots her several times and before she died, she tells Vic she thought that they could have had "something special", to which Vic replied that they did have "something special". She dies telling Vic to make sure Mary-Jo takes care of her baby daughter.
- GTA Vice City Stories
- Fear the Repo
- When Funday Comes (Boss)
- Takin' Out the White Trash (Boss)
- Marked Men (Post mission pager message)
- To Victor, the Spoils (Boss)
- Hose the Hoes (Boss)
- Robbing the Cradle (Boss)
- From Zero to Hero (Post mission pager message)
- White Lies
- Where it Hurts Most
- Lost and Found
- In the Air Tonight (post mission pager message)
- Light My Pyre (Killed)
- She has black hair with a different hairstyle as seen in the first trailer.
- She may be the only character who isn't a Protagonist that can own any car of the player's choice.