Tetsutetsu is a young man with gray hair, no eyebrows and thick pale eyelashes that encompass his eyes. Like Eijiro Kirishima, he also has razor sharp teeth.
Tetsutetsu has a tendency to act hot headed and lets his emotions get the better of him, though he also is not too stubborn to cheer for a rival, as he did so in Eijiro's match with Katsuki. He tends to have a one-track mind, and is very straightforward in his solutions to problems, as shown with his eagerness to charge straight into a fight with Mustard. He has very simple solutions to problems, and will often go about things in the most direct manner. He also tends to be vocal about his intentions.
Tetsutetsu is very determined to improve himself, as he apologized to his teacher for doing bad and vowed to do better. When villains attacked, he took the chance to expose himself to danger and grow from it. When in these fights, he is tenacious and unwilling to back down, relying on brute strength to get through rather than use much strategy.
However, he also cares for his classmates, as he didn't face the villains before making sure that he left Ibara in the care of someone else. He also threw himself in front of Itsuka when Mustard fired on her without hesitation, despite this putting himself at risk due to his own Quirks weakening effects.
Tetsutetsu Tetsutetsu first appears outside Class 1-A in a mob blocking the classrooms exit. Tetsutetsu claims he came to see the class that fought against villains at the U.S.J., but only found a bunch of arrogant brats. A member of Class 1-A, Katsuki Bakugo ignores Tetsutetsu and walks away, prompting the latter to yell at him. When Katsuki gives the pledge to begin the U.A. Sports Festival, Tetsutetsu is angered by his arrogant claim to place first and promises to crush him. 
During the Obstacle Race, Tetsutetsu uses his Steel Quirk to survive being crushed by a giant Zero-Point villain robot. He notices Eijiro Kirishima did the same using his hardening Quirk and calls him a copycat. He and Eijiro battle throughout the course, and Tetsutetsu ends up taking tenth place behind him. 
For the Cavalry Battle, Tetsutetsu teams up with Juzo Honenuki, Yosetsu Awase, and Ibara Shiozaki. At the very start of the Cavalry Battle, Team Tetsutetsu goes after Team Midoriya for thier ten million points. When they escape by using a jet pack to fly through the air, Tetsutetsu damns the support course. He later catches up to them and attacks again, but Team Mineta gets in the way and allows them to escape. Despite being in second place until the final seconds of the round, Team Tetsutetsu fails to qualify after all their points are mysteriously stolen by Hitoshi Shinso. Confused, the team tries to explain what happened. Ibara says that it was karmic punishment for stealing Minoru's headband in such an underhanded way.
The finals are revealed to be a One on One Fighting Tournament that Mashirao Ojiro and Nirengeki Shoda decide to withdraw from. Midnight has Team Kendo fill the empty slots, but Itsuka gives the opportunity to Team Tetsutetsu. Tetsutetsu happily thanks her as he and Ibara are chosen to compete amongst the top sixteen. He draws lots for matchups and ends up being paired against Eijiro. Tetsutetsu congratulates Ibara after she defeats Denki Kaminari in the third duel and to fight in the seventh match. After trading hardened blow one after another, Tetsutetsu ends up fist fighting Eijiro to a draw. Midnight decides the winner of the contest will be decided via arm wrestling contest.
Once the last match of the first round ends the arm wrestling match commences. They both use their Quirks to try and gain an advantage, but Tetsutetsu's iron gives way and Eijiro pins his arm down to win and move on to the next round. Eijiro offers his hand to Tetsutetsu, congratulating him on a match well fought. Tetsutetsu shakes Eijiro's hand and thanks him for the compliment 
Eijiro advances to face Katsuki in the next round, and Tetsutetsu roots for him during the match. He constantly chants for Eijiro to go after his chin, and is disappointed in Eijiro after he's defeated.  When Katsuki advances to the finals by defeating Fumikage Tokoyami, Tetsutetsu comments that the finals have been one big showcase for Class 1-A.Tetsutetsu joins his class during the closing ceremony of the Sports Festival to honor the winners.
Tetsutetsu receives a draft nomination from Pro Hero Fourth Kind to intern at his agency. When he arrives, he's surprised to see Eijiro there to intern as well. They happily greet each other and are introduced to the Chivalrous Hero himself. Fourth Kind explains that his agency was able to send out two nominations and he chose the two of them behind he liked their guts. He goes on to explain how wages work fro Pro Heroes, but Eijiro interrupts him, prompting Fourth Kind to yell at them both.
Sometime during their internship, Tetsutetsu and Eijiro fail to make tea to Fourth Kind's liking. He yells at them for making tea too sub par for their clients and punches both of them in the head with their Quirks active. He comments on how satisfying it is to hit their Quirks. Following the incident in Hosu City, Eijiro receives a message from Izuku and tells Tetsutetsu it was a good thing he reported it. However, Tetsutetsu is afraid of being late to the agency and tells Eijiro to hurry up.Fourth Kind catches them being tardy and yells at them again. Before the internship ends, Fourth Kind takes Tetsutetsu and Eijiro to a local park to clean up. 
When Class 1-B arrives at the training camp. Homeroom teacher Blood King explains they'll be extending their quirks and the more they use their quirks the stronger they'll be by breaking their limits. When Class 1-B witness Class 1-A's training. Afterwards Itsuka Kendo asks Shota Aizawa if can they observe everyone, he explains that's the reason the Pussycats are around.
When the Vanguard Action Squad attacks, Tetsutetsu manages to avoid the poison gas thanks to a gas mask that was given to him by Momo Yaoyorozu. He also tells Itsuka that Awase is leading some of Class 1-A to the Class 1-B safe zone for aid. Tetsutetsu gives Itsuka, Yui, and Juzo masks. After Itsuka and Yui put their masks on and Yui puts a mask on the unconscious Juzo, Itsuka thinks that they should head back to the facility. However, Tetsutetsu refuses to flee and decides to stay and fight, telling Itsuka to take care of Ibara. Itsuka questions Tetsutetsu's actions, to which Tetsutetsu tells Itsuka why he has decided to fight; they have known for the longest time that Class 1-A has always been ahead of them because they have been exposed to danger and used that exposure to gain invaluable experience in order to become better Heroes. Tetsutetsu continues to say that they will not be Heroes if they run away from evil and danger that threatens the lives of others. Tetsutetsu declares that he will expose himself to danger in order to become a Hero and promises to defeat the villains.
Tetsutetsu and Itsuka continue walking through the forest; Itsuka realizes that the gas is not dispersing and surmises that a villain is manipulating the gas; they decide to find the villain responsible for the gas. As Tetsutetsu and Itsuka reach the source of the poisonous gas, Mustard feels their presence and is impressed that they figured out the gas phenomena; he takes out a gun in preparation for their arrival. Suddenly, Tetsutetsu appears in front of Mustard and prepares to attack him with his steel fist. However, Mustard fires his gun at Tetsutetsu.
Tetsutetsu manages to block Mustard's bullet using his Quirk. Mustard shoots at Itsuka but Tetsutetsu throws himself in front of Itsuka to block the bullet. After Mustard attacks Tetsutetsu repeatedly, Itsuka attacks but Mustard evades; Itsuka enlarges her right hand and smacks Mustard away. Seeing that the gas is a hindrance, Itsuka enlarges her hands, uses them to swirl the gas away, and mocks Mustard for acting cowardly by using a gun to fight. Mustard prepares to fire his gun at Itsuka for her insult. Out of nowhere, Tetsutetsu appears behind Mustard, who was unable to notice Tetsutetsu's presence due to the gas dispersing courtesy of Itsuka. Tetsutetsu holds his breath and punches Mustard in the face with his steel fist, which crushes his mask and in the process knocks him unconscious. The victorious Tetsutetsu lies down on the ground and regains his breath.
Itsuka and Tetsutetsu head back to the facility with Mustard in tow. After Katsuki Bakugo is caputured, the Vanguard Action Squad withdraws from the battlefield, ending the attack. Mustard is arrested thanks to Itsuka and Tetsutetsu.
Quirk and Abilities
Steel (スティール Sutīru?): Tetsutetsu's Quirk allows him to turn his body into steel, which gives him a great defensive ability. He can also use it offensively to increase the strength of his unarmed attacks. Upon losing an arm wrestling match he mentions that he should have had more iron that morning, possibly implying that his quirk strength is fueled and affected by his diet containing iron.
Tetsutetsu despises Katsuki's arrogance. He has stated a desire to want to defeat Katsuki, but has not been presented with an opportunity to do so. When Eijiro faught Katsuki during the U.A. Sports Festival Finals, Tetsutetsu rooted for Eijiro to beat him.
Tetsutetsu and Neito appear to be on friendly terms, and can often be seen together. Although he didn't go along with Neito's plan to throw the qualifier in the U.A. Sports Festival, they agreed there would be no hard feelings in the Cavalry Battle.
Tetsutetsu and Ibara appear to be close friends. The two teamed up in the Cavalry Battle and partnered up in the inter-class test of courage during the training camp. Tetsutetsu cares for Ibara's well being and made sure she was safe before he went to confront Mustard.
Tetsutetsu and Eijiro were constantly compared in the U.A. Sports Festival because of their Quirks, and eventually and fought to a draw in the finals., This created a fierce rivalry between the two that wouldn't subside until Eijiro defeated Tetsutetsu in an arm wrestling match. Afterwards, Eijiro offered his hand and told Tetsutetsu he put up a great fight. Tetsutetsu accept the handshake and gained a new respect for Eijiro. ,
- His name is composed of the kanji for "iron" (鉄?), "clear" (哲?), "pierce" (徹?), and an archaic kanji for "iron" (鐵?). His name may be a joke mocking how similar he is to Eijiro Kirishima; his name sounds redundant, with each kanji being pronounced like the word for "iron" and he and his Quirk were formally introduced in quick succession and an identical situation to Eijiro. His name my also be referencing the Latin name for Tortoises (Tetsudinidae), as they are both known for being hard and resilient.
- Tetsutetsu ranked 8th during the Entrance Exam with 49 Villain Points and 10 Rescue Points.
- According to Tetsutetsu's profile in Volume 4:
- He likes fighting games.
- His favorite food is spinach.
- Kohei likes his design and wants to draw more of him.
- Tetsutetsu shares several similarities with Eijiro Kirishima.
- They share the same birthday (October 16).
- They both chose Fourth Kind's office for workplace experience.
- They share the same stats.
- Their Quirks, Steel and Hardening, are functionally similar and described likewise.
- Both have sharp teeth.
- In the anime, their Sports Festival entry poses were the exact same.
- Both their Hero names contain alliteration.
- Tetsutetsu ranked in the Top 16 of the U.A. Sports Festival First-Year Stage.
- (To Itsuka Kendo) "I'm in Class 1-B of the Hero Course! If I don't stand up here, what's the point?! I'm gonna find these jerks and beat the snot out of them!"