Assassination Classroom is funny, action-packed, and boasts an extremely unique premise. In Kunugigaoka Junior High School, the lowest-performing students, Class E, are treated like garbage to motivate the other students to succeed. They are ignored and isolated in a classroom far away from the actual school building. This makes them the perfect candidates to kill the alien octopus that blew up the moon and will destroy Earth in one year.
Korosensei is too fast and strong to be killed by any conventional military but agrees to let the kids try if he gets to be their teacher, so Class 3-E is the world's last resort. They train in the art of assassination under Korosensei himself, in hopes of killing their own teacher by the end of the school year.
Ritsu
The military assists Class 3-E in their mission, providing them with supplies, additional training, and a new classmate. The Autonomous Intelligence Fixed Artillery, later dubbed "Ritsu" by her classmates, is a weaponized AI who joins Class 3-E to try and assassinate Korosensei as well. She would not be out of place in one of the best mecha anime for beginners.
Initially an unfeeling machine, Ritsu deduces that friendship with her classmates is essential to successfully killing Korosensei, and updates herself against her creators' will to appear more human. Cheerful and earnest, Ritsu is constantly encouraging her classmates, accompanying them outside the classroom by uploading herself to their mobile phones.
Gakuhou Asano
The utterly ruthless chairman of Kunugigaoka, Gakuhou is the one responsible for the school system that runs on Class 3-E's abuse. Once a kind cram school teacher much like Korosensei, he broke down when one of his students committed suicide after being bullied, and he became obsessed with making his students strong.
Cruel and dangerously manipulative, Gakuhou is so uncompromising that he continues undermining Class 3-E for the sake of his system, despite the fate of the world relying on them. His very presence is intimidating enough that professional soldiers like Takaoka are cowed by him, as seen in the episode "Talent Time." When he's onscreen, all fun stops dead.
Irina Jelavich
In the episode "Grown-Up Time," Irina is brought in ostensibly as Class 3-E's English teacher, but also to assassinate Korosensei. As a hitwoman, she initially has no interest in the students and ignores them in favor of her assassination mission, but grows to take both jobs seriously and trains her students in both languages and infiltration.
Easily frustrated and somewhat immature underneath her femme fatale veneer, Irina is one of the show's most frequent sources of comedy. The jokes are usually on her: though Class 3-E comes to respect her, they still don't hesitate to tease her, albeit in fondness rather than rebellion.
Yuuma Isogai
As his classmates proudly claim, Isogai is Class 3-E's own Prince Charming. Persistently friendly and diligent, he is a natural choice for class representative. He wound up in Class 3-E not for poor grades, but for violating school policy by working part-time to support his widowed mother and younger siblings.
He truly shines in the episode "Leader Time." After Gakushuu catches him still at his job, Isogai will be expelled unless Class 3-E can beat Class 3-A at the sports festival. Using his lessons in history and tactics, Isogai leads his classmates to victory against the team of foreign exchange students Gakushuu brought in to overpower them, similarly to several other anime all about strategy and intrigue.
Kaede Kayano
Kaede spends most of the show in a support role until her true nature is revealed in the episode "Secret Identity Time": she hates Korosensei because she believes he murdered her sister, she turned herself into the same kind of creature he is to become strong enough to avenge her, and everything her class and the audience have seen so far is a false persona created to hide her bloodlust.
After learning that Korosensei was not actually responsible for her sister's death (and that, in fact, he was in love with her), she is shocked into a change of heart. With her desire for revenge gone, she needs something new to live for, and she finds it through her friendships with her classmates, Nagisa in particular.
Tadaomi Karasuma
As goofy as Korosensei can be, someone has to keep everyone firmly grounded. That someone is Karasuma, an agent of Japan's Ministry of Defense assigned to monitor Korosensei and his students. No matter what outrageous things are happening around him, his stern and stoic demeanor never breaks.
While Irina teaches the social skills used in assassination, Karasuma uses his position as Class 3-E's physical education teacher to train them in combat, specifically knife fighting. Though he rarely shows emotion, it is clear that he loves the students and will leap to protect them if they're in danger, leading them to consider him the "dad" of their class.
Gakushuu Asano
Gakuhou's son Gakushuu is living proof that the apple does not fall far from the tree. Kunugigaoka's all-around best student, Gakushuu leads Class 3-A with cool charisma and intense pride. He is more interested in proving his own superiority over Class 3-E than upholding Gakuhou's principles; in fact, he is at odds with Gakuhou almost as much as with Class 3-E, intending to bring his domineering father to his knees one day.
He also has a sense of honor that his father largely lacks, being willing to admit when he's been beaten where Gakuhou absolutely refuses to accept defeat. In the final episode "Future Time," he and the rest of the school's Big Five shield Class 3-E from reporters' cameras with a Kunugigaoka banner, in a gesture that acknowledges them as equals.
Korosensei
Korosensei's bright, distinctive design and bombastic personality make him very fun to watch. An overpowered creature even stronger than Naruto, he is supersonically fast and nigh-indestructible. Once the world's deadliest assassin, he gives his all as Class 3-E's teacher, helping the children realize their full potentials as assassins, students, and people.
Late in the series, it is revealed that he did not actually blow up the moon: he was once a human experimented on to make a super-weapon, and it was these experiments that damaged the moon and turned his body into a living time bomb that will destroy the world whether he wants to or not. He is deeply proud of his students and how far they've come, and truly hopes that he will die by their hands.
Karma Akabane
Though he is introduced in episode "Karma Time" as antagonistic and unstable, Karma is quickly shown to have a stronger sense of justice than his sadistic tendencies might suggest. He detests those who hurt the innocent and was suspended after attacking a bully who was picking on a Class 3-E student. Fearless and always eager to fight, Karma takes better to the mission of assassination than any other classmate, with the exception of Nagisa.
Physically, he is the strongest human member of Class 3-E, preferring to fight his enemies head-on. He is also wickedly intelligent, rivaling Gakushuu in terms of academic ability. This can at times make him arrogant and in need of a reality check, but though his failures hit him hard, he bounces back even more determined, making him one of Nobuhiko Okamoto's best characters.
Nagisa Shiota
Nobody would suspect Nagisa of being a terrifying killer. Small, quiet, and passive, he is beaten down by bullies at school and his abusive mother at home. But Nagisa takes what could be weaknesses and turns them into lethal strengths: his sensitivity to aggressors' emotions and body language makes him excellent at sensing weak points, and he is so good at hiding his presence that targets don't detect him until it's too late.
Nagisa was singled out by his teachers as having the makings of a true assassin. But instead of using his training to kill, he follows in Korosensei's footsteps and becomes a teacher. As an adult, he deliberately takes a position in a school full of delinquents because he knows he cannot be beaten down or intimidated anymore, and resolves to make as much of a difference in his students' lives as Korosensei made to his.