Thirteen's physical appearance is unknown. Their costume consists of a white space suit design using what appears to be a puffy jacket instead of an actual suit, a black helmet with white eyes, and a pair of yellow boots. It is possible to assume that Thirteen is thin, given the quite small legs showing between the costume's pants and shoes, but this is unconfirmed. Strangely, when Black Hole was used against Thirteen, the suit was empty, with strange implications.
Thirteen is a wise, well-mannered person. As a rescue hero, Thirteen has displayed how passionate they are about saving people through their teachings. They even developed their own training facility - the Unforeseen Simulation Joint - to train future heroes in search and rescue tactics. They are very passionate about Quirks, specifically on the topic of using Quirks for creation and assistance rather than for destruction and battle.
Thirteen has also been shown to be very brave. Despite not being a combat type, they do not hesitate to use their Quirk to fight against villains in the name of protecting others.
Some time ago, Thirteen was called into a meeting with Best Jeanist, Manual, Death Arms, Snipe, Shota Aizawa, Endeavor, Air Jet, Midnight, Ingenium, Gunhead and a member of the police. Speaking about the unwilling civilians injected with Trigger. The officer explained that the people in question were more of victims than villains. Endeavor argued that any public use of a Quirk is illegal and that they should treat them as villains and let the police investigate the backgrounds of future captures. He said this is the only way to protect the innocent while not risking themselves through hesitating every time they see a new enemy. All Might, who was not present, had his reply to the problem delivered by Naomasa Tsukauchi. Thirteen stated that they would assess the situation at the scene more carefully- as heroes, they must not bring harm to innocent citizens. This was in direct opposition to Endeavor's answer, greatly angering him as a result.
When Class 1-A is brought to the Unforeseen Simulation Joint, Thirteen explains that they will train there to make use of the facility's simulations of all kinds of disasters and accidents. Class 1-A's homeroom teacher, Shota, asks Thirteen where All Might is, and Thirteen replies that he was doing too much hero work to make it on time. Before rescue training begins, Thirteen speaks to the students about the dangers of Quirks and why they need to be regulated, as well as how the students will now be training to use their Quirks to save people. Thirteen concludes their speech by saying they hope everyone will understand how to wield their Quirks in rescue situations by the end of the exercise.
However, before the training exercise begins, villains appear through a Warp Gate. Shota tells Thirteen to protect the students and leaps into action. While trying to lead the evacuation for the students, Thirteen is confronted by Kurogiri, who asks where All Might is.
As Kurogiri talks, Thirteen attempts to use their Quirk without being noticed, but is unable to when two of the students attack Kurogiri. Katsuki Bakugo and Eijiro Kirishima seem to land an attack, but Kurogiri remains unphased. Thirteen pleads for the tandem to move out of the way, but Kurogiri capitalizes and uses his Quirk to separate the class by teleporting the students to several different places within the facility. Thirteen remains at the entrace plaza, along with Tenya Iida, Ochaco Uraraka, Hanta Sero, Rikido Sato, Mina Ashido, and Mezo Shoji.
Thirteen commands Tenya to use his Quirk to run to the school and alert the faculty of the attack because one of the villains is jamming his communications. Kurogiri overhears his plan and prepares to stop them, but Thirteen uses their Quirk to suck his mist body into a black hole.
Kurogiri commends Thirteen on their powerful Quirk, but also comments that they have little fighting prowess before he opens a warp-gate behind Thirteen, causing Thirteen's Quirk to backfire onto themself. Half of Thirteen's suit is disintegrated by Black Hole and they are incapacitated.
Eventually, All Might arrives, as well as the U.A. High School faculty members called by Tenya. The group manages to repel most of the villains, forcing Kurogiri and Tomura Shigaraki to try and escape. Thirteen is helped to their feet and tries to stop the villains from using Kurogiri's Quirk, but they teleport away. Afterwards, Thirteen is brought to Chiyo Shuzenji to be healed. Detective Naomasa confirms that they have recovered from their injuries when he informs the students of their condition.
During the U.A. Sports Festival, Thirteen spectates the events from the faculty section of the stands. As the battle between Izuku Midoriya and Shoto Todoroki begins, Thirteen asks All Might if Izuku and Shoto are the ones that jumped in to save them at U.S.J, which All Might confirms.
Prior to Final Exams at the end of the first years' first term, Thirteen meets with their fellow faculty members. The faculty discuss changing the finals format from facing robotic villains to fighting against the teachers. Thirteen comments that this may be too difficult for the students, but they all ultimately agree on the change. Thirteen joins the teachers at the Final Exams practical portion test site to meet Class 1-A. Thirteen is placed against Ochaco and Yuga Aoyama.
In their battle, Ochaco and Yuga reach the escape gate but before they can go through it, Thirteen stops them with Black Hole. The duo holds onto a guardrail to avoid being sucked in, thus being unable to go forward or backwards, trapped in place while Thirteen advances on them.
Yuga fires his Navel Laser from his knees (thanks to his hero costume) at Thirteen, but the attempt fails as Thirteen easily sucks in the laser. Ochaco, surprised by a comment by Yuga, accidentally lets go of the guardrail, and Thirteen stops using their Quirk to protect Ochaco from being turned to dust. Ochaco uses the unexpected momentum to turn the situation to her advantage and faces Thirteen directly. Using the combat skills she learned from interning with Gunhead she manages to pin down the Pro Hero with Yuga's assistance and handcuffs Thirteen.