Koro-sensei is a fast, octopus-like teacher in Class 3-E who threatens to destroy Earth but also serves as a nurturing mentor to his students, often changing his skin color to reflect his emotions. He is a tall, yellow creature who wears academic robes and displays a goofy, kind, yet vain personality while possessing immense, superhuman abilities. He can reach up to speeds of Mach 20+

Nagisa Shiota is the series' human protagonist and narrator. Despite his small stature and unassuming personality, he possesses a natural, latent talent for assassination. He has a slim build with light blue hair tied in pigtails, often causing him to be mistaken for a girl. Calm, observant, and deeply empathetic, he uses his quiet nature to mask his presence and strike targets when they are most vulnerable.

Karma Akabane is a highly intelligent but rebellious student known for his combat skills and mischievous nature. He is one of the few students capable of truly challenging Koro-sensei early on. He has a slender build, pale skin, and striking short red hair, typically wearing a black blazer over a white shirt rather than the standard school sweater. Proud and aggressive, he possesses a devious mind for tactics and a love for psychological warfare, masking his genuine care for his classmates behind a playful, sadistic streak.

Kaede Kayano (Akari Yukimura) is a close friend of Nagisa and a central figure in several of the series' major plot twists, often instrumental in organizing class activities. She has a petite build and light green hair styled in unique "cat ear" pigtails, usually appearing cheerful and unassuming. Beneath her bubbly and supportive exterior lies a highly determined and secretive personality, capable of extreme focus and acting as she hides her true identity and motives from her classmates.