Ritsu

Ritsu (律, Ritsu), full name Autonomously Thinking Fixed Artillery (自律思考固定砲台, Jiritsu shikō kotei hōdai), is a student in Korosensei's class 3-E of Kunugigaoka Junior High School, an AI and the first of two official transfer students of class 3-E.

Appearance
Ritsu's main body is a large rectangular robotic casing with a video screen. Inside the casing is an assortment of weaponry, as well as a 3D printer.

Originally Ritsu was just a head, however, after Korosensei modified her body and increased the size of her screen, her full appearance was shown. Her outfit changes depending on the situation; Swimwear when the class went swimming, a detective outfit when they were solving mysteries. She is most commonly shown in the winter school uniform.

Personality
Ritsu did not have any personality originally, and acted just like a robot - Focused on killing Korosensei at all costs. After Korosensei's remodeling, she gained a happy, cheery personality and learned to work with her classmates.

History
She was registered as a student in Kunugigaoka, in order to prevent Korosensei from counterattacking against her. A machine from Norway, created to kill Korosensei, she was supposed to be sent into class 3-E at the same time as Itona, but he was not ready.

Plot
On her first day in class 3-E, she fired a round of pellets at Korosensei. When he dodged them all, she added an extra bullet to her barrage, which destroyed his right fingertip. The continuous firing throughout the day irritated the other students, as they had to pick up all the pellets, as well as having to put up with gunfire during lessons. As a result, Terasaka taped her up on the second day to prevent her from deploying her guns.

Korosensei remodelled her on the end of the second, giving her a full body and an overly cheerful personality. However, her makers deemed most of the remodeling unnecessary and removed all the programs that Korosensei had attached to her. She allowed them to do so, but decided that the 'power of cooperation' was necessary for assassination, and hid its software before it could be erased.

Quotes

 * "The killing probability of the next shot is less than 0.001%. The killing probability of the shot after that is less than 0.003%. The probability that I'll be able to kill you by graduation is more than 90%."

Trivia

 * Her nickname comes from the '律' (Ritsu) in '自律' (Autonomy)
 * Korosensei spent around 100,000¥ on remodeling her, leaving him with 5¥ in his wallet. In the anime, he spent 17,009,000¥, leaving him with 5¥.
 * Approximately 985 modifications were initially made to her by Korosensei.
 * She can make anything using the special plastic inside her, excepting weapons, as long as she has the data for the object.
 * She has a mobile version of herself called "Mobile Ritsu" that debuted in Chapter 28 of the manga and Episode 10. This version is loaded onto all of her classmates' phones and allows her to travel outside of her main unit.
 * She can be counted as a foreign transfer student as she was made in Norway, thus making her Norwegian by default.
 * Her best subject is Math while her worst is Physical Education.
 * Her future goal is to reach a firing accuracy rate of 100% indoors.
 * Her special skill is manufacturing clothing objects.
 * Her social interaction software priority is Honesty while her visual display software priority is Guile.
 * As she is a computer, she is not allowed to take her tests personally. She was allowed instead to have a double take them for her, personally tutored by Ritsu herself. The substitute was later revealed to be the daughter of one of Karasuma's superiors.
 * Outside her original frame, her mobile versions' capabilities are decreased and is also prone to being hacked by skilled individuals such as the God of Death.
 * During the cops and robbers game in Chapter 82, she was codenamed "Moe Box." The one who wrote her codename was Terasaka.
 * Her proxy's name is Nise Onaga (尾長仁瀬). Her name is a pun as "Nise" is homophonous to nise (偽) which means "false" or "counterfeit" referencing her being Ritsu's fake.
 * She shares her birthday with Gakushū Asano.