The Kasumiōji Clan is a wealthy, noble family in Soul Society. High ranking nobles who didn't present themselves in public and their clan didn't associate themselves with Central 46 that much. The high ranking Kasumiōji is second to the four noble families. The Kasumioji clan has always had a female sovereign, it is an iron clad law that has been passed down generation after generation for centuries, thus a male cannot head the clan. As with all high ranking noble families the clan has immunity extraterritoriality.
They have a loyal army of assassins that serve as their bodyguards who are now led by Kekyuu Koguchi and 3rd Seat Meru Mutsumi of the Ninth Division after the previous failures of their former family bodyguards.
Years ago the steward Gyōkaku Kumoi began misusing the clans special techniques for the casting of royal swords. He exploited their ancient secrets in order for the development of the Bakkōtō. He then used the prototypes to do experiments with test subjects.
Gyōkaku Kumoi had risen through the ranks by secretly assassinating several Kasumiōji vassal Members that are loyal to Rurichiyo Kasumiōji. The Kasumiōji Clan has lost a large amount of vassals due to Kumoi's actions. After Kumoi attempted a coup against Soul Society he was killed along with all of his conspirators, restoring Rurichiyo as head of her clan and stabilizing the volatile situation.