|Spoiler Alert: This page contains spoilers. Read at your own risk.|
Juumonji Katsuto (十文字 克人) is the Head of the Juumonji Family, which is one of the Ten Master Clans. Katsuto is a university student who's attended the National Magic University since 2096. He is an alumni of First High School.
Appearance and Personality
|“||Katsuto, he who returned the cheers of victory, understood the value of that power and how to utilize it. He carried the majesty of a monarch.||”|
—Tatsuya, Volume 4, Chapter 13
Katsuto displays a very matter-of-fact persona and addresses the siblings in different ways. For Shiba Tatsuya, he refers to him as "Shiba", whereas he calls Shiba Miyuki, "Next Head of the Yotsuba". 
Katsuto is described as a rather big man, 185 cm in height with a thick chest and broad shoulders, with rippling muscles that are easily recognizable even while wearing a uniform. He also gives off an incredibly heavy sense of presence. 
Katsuto is usually passive, only commenting when necessary. However, he is honorable and dependable and has a very strong sense of responsibility, worthy to be the Juumonji family's successor. He has a no-nonsense approach as he didn’t like Tatsuya when he tried to find a way out for participating in the Monolith Code and made it clear that the decision was up to the leader and it is his responsibility no matter what to follow through. His advice to him was to stop being with the weak and using it as an excuse as he purposely put Tatsuya in precarious situations.
Tatsuya had once thought that Katsuto has a forceful personality, someone who would not bar his subordinates from fighting. Like Mari and Mayumi, he also doesn't discriminate against Weeds, which is evident when he supported Tatsuya to be one of the engineers for the Nine Schools Competition and when he reasoned with Tatsuya to accept and represent their school for Monolith Code.
Katsuto is the eldest son of the Juumonji Family, and he eventually becomes the clan head of the Juumonji. He studied at First High School. Katsuto was a member of the Cross-Field Club before becoming the Leader of the Club Management Group. Even after he left the club, Katsuto continued to use the second club room privately although this was an open secret. 
Phalanx (鐵壁) is a Move-Type Defensive Magic that isn't simply a sustained magic barrier, but a barrier that continuously refreshes itself. This is similar to a dense formation of heavy infantry that marches as one to increase the group's defensive power and then translates that power into an offense.
Even if the initial barrier falters, the one behind would simply replace it, to be continued to add infinitude. The barriers are in constant motion within set boundaries, not necessarily in front of oneself, but rather dozens of barriers can be sent crashing at high speeds into the enemy. It is impermeable to gas and can resist temperatures in excess of 20,000°C. (Note that the temperature of a "small" conventional nuclear blast, such as the one that exploded over Nagasaki, is more than 1,000,000°C at the epicentre. Modern nuclear weapons produce temperatures in the tens of millions of degrees at their epicentre.)
- Multiple barriers are erected simultaneously allowing the Phalanx to defend against a variety of attacks including firearms and magic.
- The offensive barrier which possesses the sole function of being impermeable to all attacks can even be spread out within another person's magic. The magic barrier that sets matter as its target possesses the interference strength that simply disallows any other magic to exist.
- Although there are disadvantages in that the firing range is limited and it can only be used against existing phenomena, this magic that combines anti-personnel, anti-object, and anti-magic properties has powerful applications at short distances in group combat. This magic inherited the name from the ancient Greeks' formation and displays defensive prowess and suffocating pressure worthy of its lofty name. 
It is an Area Of Effect Magic that ignores solids, fluids, and gases and creates a force field reversing motion vectors. 
The Juumonji Family took the next step to hone their natural talents in spatial awareness. Their ability to wield multiple types of Area of Effect defensive magic has earned them the title Iron Wall. With his skills, Katsuto is able to cast extremely difficult defensive Area of Effect Magics from 400 meters out. Thanks to his mastery of space manipulation and his sensitivity to changes in spatial distribution, he can read the "meaning" like Tatsuya with his Elemental Sight.
Psion Wall is a magic that the Juumonji use to place a wall of psions densely around themselves. Each time a barrier is destroyed it reconstructs itself and becomes a stronger barrier.
Although it is magic seen as a barrier type that can disable any magic it comes in contact with, it differs from the innate defense of Tomitsuka.
It is a special technique that belongs to the Juumonji clan. A technique that raises the activity of the magic calculation area in order to surpass his potential and temporarily increase his magic power. This is a technique for ensuring victory at the price of having your life as a magician shortened, the “Capitals Final Defense”.
Is a spell that uses the magic area of Gravity Control-type Magic and the Magic Area formed by the Shift Magic. As its name implies, Neutron Barrier was a type of magic that generated a field that prevented neutrons from rebounding.
Katsuto's Battle Armor looks like a medieval armor with a stury protector and helmet design similar to samurai warriors but without a face cover that he uses during the Yokohama Disturbance and his battle against Stars.