159 cm (5'3")
48 kg (106 lbs)
Nagisa has blue eyes and short, blue hair with some of it sticking out the sides of his head. His uniform consists of a waistcoat worn over a dress shirt with rolled-up sleeves and a black tie, as well as wrist bands, dress pants, and dress shoes. He has a small stature compared to the other boys in Class 3-E, being the shortest among them. He is also clearly have been mistaken as a girl when he and other Class E students are planning to get the medicine to save their classmates who have fallen sick after the drinks when they are at Okinawa Resort Island.
Nagisa is a calm and collected individual. He is exceptionally observant and circumspect of his surroundings and the situations that develop around him. He tends to pay strict attention to Korosensei in hopes to understand the motives of this mysterious being. He compiles the information he has gathered in his notes for reference.
Nagisa has been shown several times in the series to have a hidden, but also disturbingly massive, amount of natural talent for assassination that far surpasses Karma Akabane, perhaps even Tadaomi Karasuma and Irina Jelavic, despite the latter two both being trained killers, that seems to manifest itself in what can be described as almost being an alternate personality. This is most evident during his 'mock assassination' with Taka-sensei, as well as when two of Nagisa's old classmates threaten him and his raw innate bloodlust frightens both of them. This unique talent has sparked the interest of all three of Class 3-Es main faculty. When used, it seems to take the form of a large, bloodsoaked snake coiled around Nagisa's target with its fangs bared and looking like it's about to strike.
When Nagisa's mother was younger, she had been forced to study all the time by her parents so she didn't have much of a childhood. She hoped that Nagisa would be born as a female during her pregnancy so she would experience what girls would do when they were younger by having her child dress up a clothing of her choice. However, Nagisa was born a male instead. This had caused Nagisa to be abused by his mother. It was due to this reason that he kept his hair long and tie them up to prevent people from mistaking his gender. 
After homeroom is started by Korosensei, Shiota Nagisa gives the command for the class to stand. They all draw their guns, aim and fire at Korosensei while he takes the register. Nagisa listens to the creature lecture the class on their supposedly simplistic plan and demonstrates the effect the special bullets have on him if hit, much to Nagisa's astonishment. Nagisa, along with the rest of class 3-E tidy up the classroom under Korosensei's instructions.
He is present in the class after where Kayano Kaede whispers to him, pointing out the permanent crescent moon in the sky causing Nagisa to remember both the destruction of the moon and the meeting and introduction of the culprit. This happened along with Karasuma from the ministry of defense both explaining and actively showing what their future jobs will be, along with revealing the 10 billion yen reward.
Later, while thinking about how the students of class-E aren't normal students, Terasaka, Yoshida and Muramatsu approach him about working on an assassination plan, which he agrees to do with them.
During Chapter 38, Karasuma-sensei goes under the impression that Nagisa was an average student. However, during training, he felt a strange presence, similar to that of a snakes. Around Chapter 39-42, Takaoka-sensei arrives at Kunagigaoka in order to train the students better against Korosensei, challenging Karasuma to choose one student to attempt to assassinate him, using a real knife. Karasuma decides to choose Nagisa.
In the fight against Takaoka-sensei, Nagisa shows his true talent for assassination, cornering Takaoka-sensei from behind, astonishing his class and completely exceeding Karasuma-sensei's expectations.
- Main article: Karma Akabane
Nagisa is the only person that Karma listens to. They were in the same classes for all three of their years at Kunigigaoka, making them close friends. They also have similar interests, shown by their asking Korosensei to take them to the movie being shown in Hawaii.
- Main article: Kaede Kayano
Kaede and Nagisa sit next to each other in class, and are often seen together. During the Assassination Test of Courage, Korosensei paired Nagisa with Kaede.
- Main article: Ryōma Terasaka
In the beginning of the series, Terasaka suggested a plan to kill Korosensei, which Nagisa agreed to take part in. Even though the plan itself failed, it was the first suggestion of Nagisa's natural talent for assassination.
- Main article: Tadaomi Karasuma
Karasuma-sensei thought that Nagisa was a mild student with no special physical abilities. However, he did sense the presence of Nagisa's natural talent for assassination. Because of that, he chose Nagisa to challenge Takaoka in a "mock assassination", which Nagisa skillfully won.
Nagisa's physical abilities are mediocre, and he is one of the weakest among the boys. Not only that his constitution and strength is equal to a girl's. However, Nagisa has an extraordinary amount of Natural talent as an assassin; as shown in his duels against Takaoka and numerous times during the span of the story.
- (To Akira Takaoka) "I feel that Karasuma-sensei, who devotes himself to being professional, is much kinder than Takaoka-sensei, who forcefully passes off as a father."
- (To 2 Class D Students) "Kill me, huh? And yet it looks like you've never killed someone before."
- (To Kayano Kaede) "I want to know more about everyone, try killing sensei more, and live so I don't leave anything unfinished."
- He was called "Gender" during class 3-E training using code names.
- Nagisa appears as a Story Mode NPC in the PS3 game J-Stars Victory Vs, along with Irina and Karasuma
- ↑ Assassination Classroom 112 page 15-17
2.Assassination Classroom 55 : Time for the closing of the first semester