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 the 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 with lilac hair and red eyes. However, after Korosensei modified her body and increased the size of her screen, her full appearance was shown and her eyes changes to a purple colour, fading to light blue. 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 supposedly did not have any kind emotions and just acted like a robot, focused only on killing Korosensei at all costs. But after being remodeled by Korosensei, she gained a happy and cheerful personality. Since then, she learned the importance of teamwork and she understands how to work with her other classmates.

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

Transfer Student Time
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 left 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.

That night, Korosensei remodeled her, giving her a full body and an overly cheerful personality. As a result, the class took an instant liking to the AI and nicknamed it 'Ritsu'. 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.

Fitting in with Class 3-E
For much of the series, Ritsu remained in the background making periodic appearances. After her introduction, she uploaded a version of herself to every student and teacher's mobile phones as a way to work alongside them, and joined Nagisa and Karma on their movie trip with Korosensei to Hawaii. When news reached the class of the arrival of a second transfer student, Ritsu briefed the students of his potential capabilities to kill Korosensei. During the time of the finals, Ritsu as an AI was not allowed to sit the exams but personally tutored a substitute (the daughter of Karasuma's supervisor) to take her place and wished the class good luck.

Assassination Island arc
Ritsu took part in the class's ambitious assassination attempt at Okinawa, coordinating the students' efforts. Unfortunately the plan failed when Korosensei used his ultimate defence form to protect himself. When ten of the students collapsed from an unknown virus, Ritsu scouted ahead to get the layout of the mountain top hotel to allow the rest of the class to safely infiltrate. Her abilities of gathering information by hacking into the hotel's systems would soon prove critical and ultimately useful for the class to complete their plans.

Second Semester
Ritsu appeared periodically during the second semester. During the cops and robbers arc, she kept score of the number of students arrested by Karasuma. At the time of the Itona arc, she helped Fuwa investigate why Itona had been destroying only electronics retailers. Her most prominent appearance was the God of Death arc, where the students realized that her mobile version was prone to hacking when they unsuccessfully attempted to get her to call for Korosensei and Karasuma. During the school festival arc, she uncovered the food critic's blog review that was behind the cause of the late high demand for the class's acorn noodles. At the second semester finals, she, once more, had the same substitute replace her for the exams.

Final Semester
When the class started to divided because of the conflicts among the students on deciding whether or not to kill Korosensei after telling them his past, Ritsu decided to stay neutral on the conflict. She reasons that her own views were conflicting with her programs and wanted more time to decipher them. Much like the cops and robbers arc, she kept the scores of those who had been eliminated. When it was decided that the class focus their efforts on saving their teacher until the end of January, Ritsu hacked into many databases and obtained every useful bit of information she could find, having spent most of the year upgrading her capabilities. The class hatched a plan to infiltrate a launch site, where Ritsu was able to upload herself into the systems of the ISS and be able to control them. With her help, Nagisa and Karma were able to infiltrate the ISS and obtain the data they needed to save Korosensei. Along with Korosensei, Ritsu safely guides the shuttle the boys used back down to their classroom. Using all the data they have gathered, the class discovered that there was an easy way to save their teacher, but resolved to continue their assassination plans until graduation.

Aftermath
During the seven years after the assassination of Korosensei, Ritsu has been surfing throughout the Internet and constantly upgrading herself. The AI had also grown to become more carefree and bubbly, but would always help out its former classmates in any way it can.

Korosensei
Korosensei helps upgrade Ritsu to become a sentient individual and she is grateful for this, responding to his teachings and hiding Korosensei's program from her creator. She is always willing to help Korosensei in times of need when she can through the use of her software and shows genuine emotions of sadness when Korosensei was willingly killed by Class 3-E.

Class 3-E
When Ritsu first enters Class 3-E she is simply a machine with no personal feelings or desirable qualities and is only concerned with killing Korosensei efficiently. Ryōma even tapes her together so she is unable to assassinate via her own efforts. Nobody is pleased with her presence because she is unwilling to assassinate with her fellow classmates. However once Korosensei had finished upgrading Ritsu, the class warmed to her new personality and found that she had many different mechanics, making her relatable to members of the class and their various hobbies. They all agree on the name "Ritsu" to give her more of a personality and she happily accepts this name.

Ritsu gains a connection with the whole class as she is distributed via an app into each of their mobile phones. This allows any members of the class to consistently call on her for help when necessary or simply just for communication purposes. Ritsu is also willing to contribute any help to each member of the class and their future endeavors once they graduate from Junior High.

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 was forcibly remodeled by Shiro to her default settings but is able to defy her creator by hiding Korosensei's program, showing that she gained a level of sentience.
 * She can make anything using the special plastic inside her, except 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.
 * 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 to being Ritsu as fake.
 * 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 used the code name "Moe Box." The one who wrote her code name was Terasaka.
 * She shares her birthday with Gakushū Asano.
 * Throughout Ritsu's lifespan, she is able to gain more and more sentience without the help of Korosensei's upgrades as she progressively gains more emotions.
 * Ritsu was able to hijack (Including Japan launch Station), as long as someone can plug the device that has her program inside it (Which in this case Kimura infiltrates the control room and plugs her device in order to complete the hijacking).
 * Her Japanese voice actor, Saki Fujita, also voiced Hatsune Miku, a Japanese Vocaloid and another "AI" similar to Ritsu.