The students sure seem to like shaking their hands around during season one's openings. But that's not all; if you take a closer look at their hands, they form two gestures: a closed hand moving back and forth (as if holding a knife and stabbing someone), and an open hand (a gesture used for calling a teacher's attention to either ask a question or to answer). It is a neat symbolism of their relationship with Koro-sensei, who is their target AND their teacher.
Irina and Ritsu became part of the first opening as soon as they became part of the assassination classroom cast. But Karma, unlike the former two who were previously absent in the opening, was there from the start. This is because he was already a student at Class 3-E, thus already an official member of the cast. His only reason for not being present in the until the end of episode two was because he was under suspension.
Some of Koro-sensei's weaknesses can be inferred or justified by his personality, actions, or another weakness.
Weakness 12: Getting wet. During a particularly humid day, his head absorbed the moisture in the air and swelled up. A later chapter reveals Weakness 23: He cant swim. His entire body will swell up when exposed to water.
Weakness 29: He gets carried away by official positions, where he took his role as the jailer during the Cops and Robbers game way too seriously. Earlier, Koro-sensei also played the role of a hot-blooded baseball coach and an overly strict lifeguard (Weakness 22: Hes very strict about poolside manner) to the same degree. But what's Koro-sensei's official job, you know, the one that he's really good at and the primary motivation behind playing all of the above positions so well? Being Class-E's homeroom teacher.
The amount of weaknesses Koro-sensei has and how he can't truly overcome them? Makes complete sense once his backstory is out and it's revealed that, contrary to Itona's request to be strong, he asked to be weak.
Nagisa is always wearing two thin, black bracelets around his wrists. After Chapter 112's reveal, it's safe to assume that they're actually back up hair ties. He really doesn't want to be seen in public with his long hair down.
The Chapter 125 Colour Page has Kayano and Koro-sensei on it for Valentine's Day, both of them thinking about the same type of candy in the snow, and a Valentine's candy box with "Got to kill soon" written on it, with the "kill" written over "meet". Then along came chapter 128: Kayano has tentacles and wants to kill him. In hindsight, it was like a knife to the reader's throat and we never caught on until The Reveal blew it wide open, like a true assassination - we never saw it coming until it already occurred.
Starting from Episode 7, when the class leaves for the Kyoto field trip, Ritsu appears at the end of the opening theme even though she isn't seen or formally introduced until Episode 9. It only makes sense when you realize that she did in fact made her classroom debut during Episode 7. She was being installed there while the students were away on the field trip.
In chapter 133, Korosensei starts revealing his back-story by stating that he was once an assassin known as the "God of Death" two years ago. All of a sudden, all of the advice he gives and all of the lessons he taught to his students about assassination makes sense - he was once THE Best Assassin In the World,, so of course he'd be the best teacher for a class about assassinations, particularly his own.
It also goes a long way to explain why, in the lead up to the God of Death arc, the initial victims were Only Mostly Dead. The one who attacked them was a skilled but significantly less competent copycat or inferior successor.
The fundamentals of the God of Death's invisible scythe technique were even used by him in the flashback, with grains of sand.
Ever since reading the title, I was really curious. It was revealed since the first chapter they were trained to kill the teacher, but isn't an assassin would be more suited to teach about assassination? Then Bitch-sensei came, and I am a bit disappointed after her introduction. And then there's the reveal in chapter 133. It really IS an Assassination Classroom, where the children are taught to kill by the best assassin himself.
Even better, with Irina they finally have teachers covering all facets of assassination. Karasuma works their physical abilities while Irina, one of the best Honey Trap style assassins in the world, trains their social skills so that they can manipulate people and get close to their target. Koro-sensei's true strength as the God of Death was that he was a Master of All polymath. Fittingly for their form teacher, he hones their mental skills and improvisational ability. His lessons often involve playing on their strengths and thinking their way through the assassination. Really, it is as good a class for raising assassins as anyone could ask for.
Back on the page for The 47 Ronin, it mentions that a lot of the characters in Assassination Classroom are named after them, with Koro-sensei being an analogue to Kira. the oft mentioned question of the possibility of Kira dying of natural causes before the actual assassination happens is finally given an analogue through the simple fact that Koro-sensei could eventually meet his end in such a fashion if the students don't kill him themselves by March, with the rest of the world hanging in the balance as well.
The bit of Early Installment Weirdness where Koro-sensei actually threatened everyone's families to hammer home how serious is about how they cannot endanger each other in their mission? Considering his mannerisms are based off of Yukimura Aguri and this is early in on the year, he hasn't fully embraced his new persona. And, on top of that, having someone nearly get killed while hugging him would strongly remind him of Aguri's death.
In Kayano's spotlight chapter, she builds a giant, bomb-concealing pudding for the purpose of killing Koro-sensei. The other students help her, and a good time is had by all. Finally, just Koro-sensei is about to eat the finished pudding, Kayano flips out- she has so many good memories associated with that pudding that she can't bear to see it destroyed. The same thing happens dozens of chapters later with her 'false' persona, which was created for the same reason.
Femme Fatale Irina is (somewhat) religious, making her a literalpraying mantis.
Which student was it who ultimately "killed" the threat that Koro-sensei was for the Earth? It was Okuda, who politely ASKED for her teacher's help in her attempt to do so. Of course Koro-sensei would make sure that was the best assassination ever. And the Anti-Matter was destroyed.
The fact that Koro-sensei had the students kill him instead of the laser. As all the suspicion being placed upon them would quickly disperse. Since they killed the "monster" who allegedly brainwashed them causing them to be viewed as heroes.
Nagisa in the final has not grown an inch since middle school, his parents never got back together, and is placed in a, possibly dangerous, classroom full of delinquents. He still makes it work, which works perfectly for the biggest character and ultimately The Protagonist of the series. No, he didn't get exactly what he wanted, but he can keep going anyways and manage to succeed...something that is a good thing to show to the readers.
Kayano's obsession with big boobs? It's probably a symptome of the tentacles, as both Korosensei and Itona were shown obssesing about big boobs.
Koro-sensei's "as a fellow ikemen" comments during the arc about Isogai's part-time job are kinda out there and played for laughs. As the old God of Death, he actually does fit the description of an ikemen rather well.
A sad example that still impresses me with what a good character Nagisa is. In the earlier chapters, his skill at assassination just seemed like a cool ability for our protagonist. But once it's revealed that his mother is deeply abusive, it's clear that his "superpower" is really his hypersensitivity towards people's emotions that he developed while trying to protect himself from his mother's anger. It really adds a layer of depth to the standard "the hero is just special", and makes his application of his ability by the end of the manga even more heartwarming.
How come Kanzaki didn't get a haircut during the taboo game? She's either been taught to be really kind by her really strict parents or it's because of her experience as a gamer during her delinquent phase. Turns out she is that good at games.
In chapter 95, Koro-sensei snaps and slaps his students because their reckless and arrogant use of their athletic abilities made them injure an old man; Koro-sensei specifically accuses them of forgetting to "think from the standpoint of the weak". In chapter 140, we learn why that event enraged him so much: that flawed mindset is exactly why he couldn't prevent Aguri's death. In his life, he only cared about gaining "strong" abilities - those that could be used to kill others; he lacked any ability that could've been used to help the "weak", like the medical knowledge to heal Aguri's wound. During his rampage, he was completely focused on the "strong" enemies, so he ignored the "weak" Aguri and failed to protect her from the tentacle mine, causing her to get get that fatal injury.
Nagisa learns the "stun clap" technique (clapping when the opponent is highly stressed, to distract them for a critical instant or even to paralyze them), and initially wants to be an assassin. Later on, he develops a "calming touch" technique (touching someone in the neck when the're highly stressed, to distract them and snap them out of their panic) to calm down Sakura, and that event is what makes him decide to become a teacher instead. Both of these techniques are based on a similar principle. In the manga, the first chronological usage of these techniques is the original God of Death - an assassin - knocking out Yanagisawa by hitting a glass pane; the last is Koro-sensei - the original God of Death turned into a teacher - calming down Nagisa.
Notice in chapter 149 Nagisa didn't release Karma until Karasuma announced the surrender formally? In fact, Nagisa was still holding on with all his might, choking Karma until he learned his side had won. This makes sense when you realize Karma is a dangerous Blood Knight who doubles as a bit of a Manipulative Bastard. Nagisa has seen Karma repeatedly beat people up and very rarely plays fair. During Karma's fight with Grip, he fights them evenly before faking being knocked out in order to finish off the assassin. Nagisa probably believed that that was what Karma was doing; trying to get Nagisa to release him so he could win. Nagisa, unlike the vast majority of Karma's opponents, has known him for a long time, and as such is used to Karma's tricks. There was no way he would take any chances until the surrender was for certain.
The three main human characters of Assassination Classroom, Nagisa, Kayano and Karma all have several things in common if you notice. The first would be that they all have pretty unnatural hair colours, Nagisa's being blue, Kayano's being green and Karma's being bright red, though Kayano's hair is dyed. Another is that there is something weird about their names; Nagisa was named specifically so his mother could think of him as a girl, Karma's name is pretty weird on it's own (though he states he likes his name and asks to be called it over his surname), and Kayano's name isn't even real, her name is actually Akari Yukimura. The last similarity is that all three have attempted to kill Korosensei in a way that basically doubled as suicide; one of Nagisa's first attempts was by wearing a small grenade while hugging Korosensei that would have killed him if not for Korosensei's intervention. Karma explicitly stated that his Xanatos Gambit would end with either both him and Korosensei dead, as he would shoot Korosensei as the teacher tried to save him from jumping off a cliff. Or Korosensei would 'die' as a teacher for letting Karma kill himself. However, thanks to Korosensei's quick thinking, neither ended up dead. In Kayano's second assassination attempt, she used tentacles she implanted in her neck that very obviously harming her body. Itona even stated that she would have only survived about a minute or so like that. Luckily, Nagisa managed to save her via kissing.
In the third opening of the anime, Question, you can see characters with certain relationships to each other shown side-by-side; Professor Bitch and Karasuma, and Principal Asano and Gakushuu. However, the Reaper appears all by his lonesome with what appears to be only his silhouette behind him. If you notice, this silhouette, though vaguely, resembles Korosensei while he was human, which would make sense, considering the nature of their relationship.
Nagisa and Karma appear next to each other as well in the opening. This can be seen as foreshadowing towards the deterioration and restoration of their friendship in later episodes.
During the intro to the Hotel Arc, the mastermind demands that Nagisa and Kayano trade off Koro-sensei for the antidote. Karasuma considers complying, despite the risk of a worst case scenario where both kids are kidnapped and the antidote is never handed over, but eventually decides against it. After reading the entirety of the arc, you'll realize just how horrific the actual "worst case scenario" would've turned out:
As detailed during The Reveal, Kayano and Koro-sensei would have been restrained and buried together in anti-sensei material and cement. Koro-sensei could either escape by exploding, killing Kayano in the process, or save her life by sacrificing himself and succumbing to the anti-material. Though there's no guarantee that the second option would've been explosion free...
Meanwhile, the same events would've occurred for Nagisa: Takaoka would have trapped him on the heliport and blown up the antidote, sending Nagisa spiraling into his Unstoppable Rage—except Terasaka wouldn't have been there to throw him the stun gun and snap him out of his bloodlust. Without a clear head and the second weapon, Nagisa would not have been able to utilize the secret technique, rendering him powerless to stop Takaoka from killing him. Or, to take Takaoka's words at their horrifying face value, from killing him very, very slowly.
With the reveal in Chapter 128, it can also be inferred that The pool of anti-sensei material would have also killed Kayano anyways. Korosensei would have been in a situation where there was likely no chance to save Kayano, so he would have died knowing one of his students would have died with him.
Nah, Kayano won't surrender quietly, it's more likely she'd be forced to reveal identity and defeat Takaoka at that.
...or sacrifice herself in order to kill Koro-sensei.
In chapter 107, we see more of Irina's backstory and her start as an assassin. There's a particular panel where she describes her second ever job, which depicts her mostly-nude in bed slitting the throat of a man pushing her down. Based on how young she looks and the fact that it was so early in her career, she probably became a "honeypot" disturbingly early...
Koro-sensei's backstory reveals he asked to be weak during his transformation. This guy who can go Mach 20 and is nigh unkillable is weak. The amount of power one might have if they went through the same process and asked for strength is horrifying with that in mind.
Itona Horibe and Akari Yukimira desired strength and both went crazy with their new powers as a result of the tentacles absorbing a ton of energy from the brain in order to grant them this power. If Koro-sensei asked for the same thing, he would have been an insane monster hellbent on killing everyone on the planet.
And it seems like the second Reaper has become just that as of Chapter 157.
Yanagisawa/Shiro has repeatedly declared his intention to give Koro-sensei "the cruelest death imaginable." In Chapter 170, he and the second God of Death show up, interrupting a heartwarming moment between Koro-sensei and his students and revealing that "Number 2" has been transformed into another tentacle monster. Past assassins, including Number 2, have noted that using Koro-sensei's concern for his students against him is an extremely effective tactic. Now, under those circumstances, what could possibly be the cruelest thing to do to Koro-sensei...?
Behold chapter 173.
The Chairman's heart, while twisted, was in the right place by wanting his students to come out from his teachings stronger mentally and physically, thanks to his previous student's suicide. But with his emotionally exhausting teaching methods, his brainwashing, and his strictness, who's to say that somewhere down the line his students won't come out of there suicidal as well?
It could be chalked up to Early Installment Weirdness, but late in the story Koro-sensei reveals that his tentacles make him fast but not correspondingly strong, and in terms of his actual strength he isn't that much stronger than a human being. Except in early arcs he was shown casually handling anti-air missiles, even holding one in place perfectly when its rocket fired, without any apparent effort. That would require a tremendous amount of strength. One has to wonder if he made up that weakness entirely.