Katsuto Jūmonji is heir apparent to the Jūmonji family, one of the Ten Master Clans governing Japan's magicians. The Jūmonji family wields significant influence in the Kantō and Izu regions, participating in regional monitoring and status reports during Master Clans Conferences. Katsuto has represented his family at these meetings, including substituting for his father, Jūmonji Kazuki, during his father's health absences.

He attended First High School as a senior and led the Extracurricular Activities Federation. Alongside Mayumi Saegusa and Mari Watanabe, he formed the school's "Big Three," top magicians renowned for their abilities. His magic specialty is Phalanx, a Multiple Move-Type Defense Magic. This technique constructs psion walls to block physical and magical attacks, serving both defensive and offensive purposes. Phalanx operates within the physical dimension, intercepting magical energy before manifestation, but cannot inherently defend against spiritual phenomena or mental interference attacks originating outside that realm.

During the Visitor Arc, Katsuto participated in investigations of paranormal "vampire attacks" linked to entities known as parasites, a threat addressed through collaboration between the Saegusa and Jūmonji families. He assisted allies against magicians from the USNA's STARS unit, intervening when operatives Ralph Algol and Ralph Hardy Mirphak attacked during the Offshore Tower incident. His Phalanx defense notably withstood Tatsuya Shiba's decomposition magic until overwhelmed by a specialized countermeasure, demonstrating its resilience.

The Jūmonji family ranks among the top three most influential within the Ten Master Clans, following the Saegusa and Yotsuba families. Their responsibilities include contributing magical support during national crises and preventing misuse of magic, adhering to doctrines mandating strength and control among Japan's magicians. Family heads and their status are widely recognized within Japan's magic community.