Earth is threatened by an enormously powerful octopus-like monster who destroyed 70% of the Moon, permanently rending it into the shape of a crescent. The monster claims that within a year he will destroy the Earth, but he offers mankind a chance to avert this fate. In class 3-E; the End Class of Kunugigaoka Junior High School, he starts working as a homeroom teacher where he teaches his students regular subjects, as well as the ways of assassination. The Japanese government promises a reward of ¥10 billion (i.e. 100 million USD) to whoever among the students succeeds in killing the monster, whom they have named "Koro-sensei" (殺せんせーKorosensē, a pun on korosenai (殺せない, unkillable) and sensei (先生, teacher)). However, this proves to be an almost impossible task, as not only does he have several superpowers at his disposal, including the capacity of moving at 20, but he is also the best teacher they've ever had, helping them to improve their grades, individual skills, and perspectives for the future.
As time goes on, the entire situation gets even more complicated as other assassins come after Koro-sensei's life, some coveting the reward, others for personal reasons. The students eventually learn the secrets involving him, the Moon's destruction and his ties with their previous homeroom teacher, including the true reason why he must be killed before the end of the year term.
Nagisa Shiota, a petite-framed boy with blue hair and azure eyes. His hair goes past his shoulders due to his mother forcing him to keep it that way. He kept it in a long ponytail until joined the classroom and put it in two pigtails. He is known for his androgynous appearance which has often been poked fun at by many people. Due to his small frame, soft voice, and peculiar hair, he's easily mistaken for a girl. Seven years after graduation, his hair is noticeably shorter and no longer tied in pigtails, though he still keeps his bangs. He retains his signature uniform waistcoat even he already become a teacher of Paradise High School. Nagisa is a calm, collected, and friendly individual who can be exceptionally observant and circumspect of his surroundings and the situations that develop around him. He tends to pay strict attention to in hopes to understand the motives of this mysterious being and compiles the information he has gathered in his notes for reference. Nagisa is noticeably more reserved and shy than other students in his class, thinking much more than actually talking and referring to himself as "invisible" because of this. He seems to be commonly nervous about what he says in important situations, being hesitant and harboring a slight stutter, which is possibly due to his mother telling him he "isn't good enough". However, Nagisa isn't easily fazed by things that would unnerve the average person, as shown with his interactions with Karma and the prospect of using a real knife. Despite his calm and empathic demeanor, Nagisa harbored deep insecurity about his talents and how he was perceived by the people around him. This was reflected by a lack of goals and general pessimism about his own future, which caused him to be put in Class 3-E. However, Nagisa eventually gained more self-esteem, became even more objective, and found his own goals thanks to his experiences with his classmates and Korosensei.
Rio Nakamura, long, straight strawberry blond hair (blond in the anime and dark blond in the OVA) and grey eyes (light blue in the anime). She wears the Kunugigaoka school uniform with the assigned yellow sweater. Rio is a friendly, energetic girl and somewhat of a trickster. She's playful, has a bit of a dirty mind, and has been noted to be rather intelligent. She claims she was originally a serious person, something that would prove to be true when she disagreed with Nagisa's proposition to save Korosensei and continue to kill him.
However Nagisa and Rio grew closer time went on in the class and soon they each had their own feelings for each other but they couldn't confess to each other out of fear that the other to reject them, but at the end of the year after their graduation:
Nagisa called over Rio to the outside of the building as they just stood there in silence and awkwardness until Nagisa spoke up
Nagisa: "Hey Rio, can I ask you something?"
Rio: She blushed as she heard what Nagisa said "S-s-sure, what is it?"
Nagisa: "Well.............." he takes out a letter with a heart and bows to Rio handing it to her "I love you, Rio, I loved you for a long time so please accept my confession."
Rio: She takes the letter "You know.... I actually like Karma."
Nagisa look up with a heartbroken face only to be kissed by Rio as she put a finger on Nagisa's lips before he could answer
Rio: "Just kidding, I love you too" She kisses him again and Nagisa kisses back as they explored each other tongue's as they release leaving a string of saliva
Nagisa: Look surprised and shocked "T-t-that was-"
Rio: "Amazing" She smiled "Now we are official, but we can't reveal it yet, so for the meantime, we are dating in secret ok?"
Nagisa: "Ok then, I love you Rio" He smiled
Rio: "I love you too Nagisa" She smiles
