Brittany is Brooke's sister. She is not in The Powerpuff Girls, or the Powerpunk Girls.
Created by an unknown source, Brittany remained completely anonymous for years, living in an abandoned subway station.
During the war, Brittany acted as a guideline for Boomer, leading him to the discovery of his Brothers' disappearances. To make Boomer for confidant in himself, she gave Boomer a part of each quest, and made him figure out the rest of it himself.
During the reform, Brittany was falsely accused dead, after being knocked off of the cliff at Sadona-Tay beach by the Powerpuff Girls, leading to...