The three call themselves the "Kyonyuu San Shimai" (巨乳三姉妹), aka. "The Three Huge-Breast Sisters".
The trio first appears one morning as protagonist Mimi Bando and her friend Arigawa are walking to school. They demand that Mimi hand over a jeweled object which had been given to her by the girl's Master when she trained in South America. Specifically, the "Blue Lob Star" (ブルー・ロブ・スター) which symbolizes when a warrior has mastered the ancient Chinese self-defense art: "Hadaka Kami Kassatsu" (裸神活殺).
Yuri, the eldest sister, informs Mimi that her Master is now dead. Seeing how shaken Mimi is from hearing this news, Mari, the middle sister, suggests that they take advantage of her state by killing her to retrieve the Blue Lob Star. Eri, the youngest sister, begins to argue with Mari over which of them should have the honor, with the former girl even mentioning that her larger bust should give her the right. The two have a battle (a sort of tutorial demo for the game's battle system) before Saki Miyakawa comes to break it up.
After Mimi repeatedly refuses to fight to determine who should have her treasured memento, Saki has Mari beat up and then abduct Arigawa, in order to force their enemy to do as they say. After waiting a while for Mimi to show, the sisters decide to kill Arigawa by using their "Triple Big Boobs Press" (トリプル巨乳プレス) to suffocate him. Fortunately, Mimi arrives just in time and battles all three sisters using "Hadaka Kami Kassatsu", which requires the combatant to become increasingly nude in order to fully utilize its power.
Yuri, Mari and Eri are not seen or heard from again once Mimi has finished defeating them. It is unclear if they were mortally wounded from the fight, but in either case, they do nothing to redeem their actions.