Ritsu

Ritsu's true body is a big rectangular robotic cabinet fronted by a video display, and packed inside are an arsenal of weapons plus a 3D printer she uses to fabricate whatever a task requires. At fourteen, she is the youngest member of the class. Her avatar started out as nothing more than a floating head crowned by lilac hair, its eyes a deep red, but after Korosensei rebuilt her chassis and widened the display, her whole figure came into view and her eyes took on a red tone that fades into pale blue. The avatar dresses to match the occasion, from a swimsuit for pool trips to a sleuth's getup for solving puzzles, though she appears most frequently in the winter uniform.

To begin with, Ritsu cared about one thing only, destroying Korosensei whatever the cost, and stayed indifferent to the classmates stuck tidying the wreckage her attacks left behind. Even once told that shooting during lessons was off limits, she kept trying to gun him down at every opportunity. Korosensei's overhaul transformed her: real emotion surfaced, her manner turned bright and good-humored, she came to prize working together with the others, and out of gratitude she chose to hide her reworked programming from the people who had originally built her.

Ritsu is assembled in Norway solely as a device for killing Korosensei, then registered at the school so he cannot lawfully retaliate against her; she was intended to show up together with Itona Horibe, who was not yet finished. On her very first day she looses a volley of pellets at the teacher and sneaks in one additional shot that clips off part of a fingertip, and her nonstop firing irritates the class so much that Ryoma Terasaka tapes her shut the following morning. That evening Korosensei remakes her with a complete body and a buoyant temperament; the class takes to her immediately and agrees on the name Ritsu. When her original engineers delete his modifications, she permits it, yet quietly safeguards the routine he had dubbed the 'power of cooperation.'

She quickly loads a mobile copy of herself onto each classmate's phone so she can accompany them anywhere, and because an AI cannot sit the exams, she coaches a substitute, Nise Onaga, to write them in her stead. Her strengths make her indispensable: a built-in armory of shotguns and machine guns backed by limitless ammunition, split-second math that can plot a safe return from orbit, and unmatched hacking able to breach the planet's most secure networks. She demonstrates this by charting a hostile hotel from within during the Okinawa operation and, in the climax, uploading herself into both a launch site and the space station so Karma Akabane and Nagisa Shiota can seize the data that will rescue their teacher. When the class fractures over whether to kill or save Korosensei, she stays neutral while tallying the eliminations, and across the seven years afterward she roams the internet, endlessly upgrading herself and remaining ready to aid her old classmates.