The rivalry occurred between the East Japan and West Japan in the world of Keijo which has been occurred for more than ten years.
In the professional world of Keijo, the title Prize Queen is currently occupied by Fuyuyu Tsurugi, an East player. Additionally, all of the top five strongest Keijo players, the Five Butts, are East players. Furthermore, they have been managed to maintain their position for the past several years. As a result, many tournaments are usually dominated by East players. The rivalry can also be seen among the branches. For instance, a few players from the Hyougo Branch dislike East players. Meanwhile, players from the Funabashi Branch are hostile towards West players.
In order to restore the balance between the East and West, Fujisaki Family plans to take down the Five Butts by inventing a strengthened swimsuit called the "Unlimited Panty". On the other hand, an S-Rank player, Suzuho Kato founded an organization called the West Butt Coalition to gather various West players with the aim of defeating the Five Butts. Outside of matches, many sponsors tend to support East.