Brooke, a member of the wealthy Armstrong family, is the daughter of Hayley and Laurel Armstrong. She was introduced in the third season as a new employee of the D&D advertising agency. Though she initially showed a pleasant side, Brooke would gradually reveal a cocky, snobbish, malevolent and mischievous nature as well. A love triangle eventually developed between Brooke, Billy, and Alison, with Brooke manipulating Billy into thinking that Alison was no longer interested in him. She also used her father's power in the company to have Alison relocated to Hong Kong.
After Alison realized the extent of Brooke's actions, she arrived at Brooke's wedding to Billy and attempted to stop the ceremony. Billy, however, decided to go through with the marriage.
Brooke and Billy later believed that she'd become pregnant, and Billy was overjoyed. However, Brooke soon learned that she in fact was not carrying a baby, but let Billy continue to think that she was. After Billy learned the truth, he became outraged and declared that their marriage was over. He later changed his mind after Brooke attempted suicide after he left her, but was soon repulsed by her actions once again after she terrorized Alison.
A drunken Brooke drowned in the Melrose Place pool after hitting her head during a fall. Though he was initially distant, Billy honored her memory by saying a final goodbye at her grave. Brooke was referred to by Jane and Samantha in Season 5, when Billy was wedding with Samantha in Season 6, and when Amanda sold the apartment complex in Season 7.