Cementoss appears to be a humanoid block of cement with black eyes and gray hair tied into a ponytail. His body shape is evenly blocky all around his body. His eyes are generally closed and his lips are pulled upwards. His hero costume consists of a sleeveless brown suit that includes yellow stripes, shorts, and an elongated collar. He also sports gray shoes with yellow soles.
Cementoss is an often serious, yet pleasant individual. He is very careful of the well being of others. He did not allow Eijiro to compromise All Might's identity and also wanted to stop the match between Izuku and Shoto, noting that Izuku's injuries were serious enough to stop the bout.
Cementoss first appears as a part of U.A. High's judging panel during the Entrance Exams. He is visually surprised when Izuku Midoriya uses his Quirk to destroy the giant robotic villain in a single punch. 
Cementoss arrives with his fellow faculty members when Tenya Iida brings them to the U.S.J to halt the League of Villains assault. He uses his Quirk to create a wall between All Might, Izuku and Eijiro to protect All Might's identity. He tells Eijiro it's dangerous and commands him to return with his class. All Might thanks Cementoss for this, and Cementoss replies that he's actually a big fan of the number one hero. 
During the U.A. Sports Festival's final event, Cemetoss acts as the proctor for the one on one fighting tournament. He creates the ring where the combatants will fight and a chair for himself to sit. He sits down and warns the participants that he will step in if things get too rough. 
During the fight between Izuku and Shoto, Cementoss contacts Midnight and asks her if they stop their fight because Izuku's fighting recklessly. He adds that his adrenaline is blinding him from how serious his injuries are and that Recovery Girl likely won't be able to heal him in a single session. He also says Izuku may not even be able to fight in his next match even if he wins. They allow the two to continue fighting until the climax of their bout. Worried that the two may kill one another, Cemetoss and Midnight step in. However, Izuku's and Shoto's attacks hit Cementoss's cement pillars and creates a shockwave that destroys the entire arena. After the smoke from the collision clears, Cementoss comments that although great power isn't always a good thing, he find Shoto and Izuku both amazing. 
Following the U.A. Sports Festival, U.A. students receive internship opportunities from Pros hoping to draft them. In the faculty room, Cementoss informs All Might that a new nomination has arrived for Izuku.
Prior to Final Exams at the end of the first years' first term, Cementoss meets with his fellow faculty members. They discuss changing the finals format from facing robotic villains to fighting against the teachers. Cementoss comments that this may be too difficult for the students, but they all ultimately agree on the change. Cementoss joins the other teachers at the Final Exams practical portion test site to meet Class 1-A.Cementoss is placed against Eijiro and Rikido Sato in the first match.
The three of them arrive in a replica city area where their battle begins. Cementoss begins by using his Quirk to create many cement walls to block their advance from the front. Eijiro and Rikido use their Quirks to smash their way through the walls, but Cementoss continues to create more. As both the student's Quirks work on a limit, Cementoss holds the advantage in a drawn-out battle. Cementoss informs the duo of their weakness as he overwhelms them with cement and knocks them out. 
Cementoss along with All Might, Ectoplasm and Midnight, were the repossessed by training class A and B students to develop their Quirks and special techniques before the provisional hero license test.
Quirk and Abilities
Cement (セメント Semento): Cementoss' Quirk allows him to manipulate any cement based material that he touches, allowing him to shape it into various forms. His Quirk doesn't appear to have a time limit, and Recovery Girl mentioned his Quirk is amongst the most difficult of the U.A. teachers to fight against. 
- "Ken" (堅?) means "stiff, firm, hard". "Ishiyama" (石山?) means "Stone Mountain". "Cementoss" is possibly a combination of "cement" and "toss".
- According to Volume 4:
- He's a big fan of All Might.
- He likes round things (whilst being, ironically, blocky).
- His favorite food is manju.
- On his profile in Volume 4, the author commented that he should have given Ken's hair a crew cut.
- Cementoss shares his first name with Ken Takagi, the Pro Hero Rock Lock.
- (To Eijiro Kirishima and Rikido Sato) "Listen. When it comes to fighting... It's all about making use of your particular strengths."
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