Koro-sensei's interaction with Karma at the end of chapter six/Episode 3, helping Karma take a turn for the better.
Episode 3, Koro-sensei lets Karasuma know that he prefers the name his students gave him, and so insists that "it's Koro-sensei to you."
Many of Koro-sensei's interactions with the students are definitely this. Some big ones include Koro-sensei protecting Nagisa in Chapter 1, then in Chapter 2 when he helps Sugino realize why he is struggling with baseball, then again in chapter 7, with Okuda who had trouble with anything but math and science. Hell, at one point, when Nagisa and Karma wanted to go with him to catch a movie in Hawaii, he was more than happy to oblige. He even gives them an extra lesson on the way!
In Chapter 11, Assembly Time, the Student Council purposefully didn't bring enough schedules in order to humiliate Class E. Koro-sensei quickly HAND WROTE and distributed copies for each of his students.
And later on, when Nagisa is being bullied, Koro doesn't intervene specifically because he knows that Nagisa won't be intimidated by the likes of the higher-ranked students. He's so confident in this fact that he shows his smug striped face. And he's right.
Chapter 10, when Irina insults the students and tells them to not bother studying for High School exams (because she claims they will more than likely fail) the students go berserk and demand Koro-sensei back. This is actually rather heartwarming and awesome when you realize they actually do like Koro-sensei and actually like his lessons (or at least prefer him over her).
When the rest of the class expected Ritsu to take out her guns she, instead, took out four bouquets of flowers that she promised to make, proving that Koro-sensei's upgrades allowed her to obtain free will and disobey her creators.
To be more specific, Koro-sensei upgraded her overnight to allow her to recognize the students' placements and co-operate with their own efforts in assassinating Koro-sensei, as before she simply blasted away at Koro-sensei without actually minding them. Her creators caught wind of this within the day and stripped her down back to before, even going so far as to reset her programs to factory defaults and even stating that upgrading Ritsu constituted as "harming" her. Ritsu, however, recognized that co-operating with the students was the best solution, and so stored the program upgrades Koro-sensei had given her within another program - and she kept her promise to show the students flowers using the same material her guns are made from. The anime's adaption of the arc helped sell this last point simply by way of the flowers being animated.
Chapter 14 (episode 6): Test Time.
When the students claim that they don't care much about midterms because they still have the chance to earn the money from killing him, Koro-sensei builds them a proper sports camp and tells them that he wants them to show their intelligence and gain the knowledge they need for the future.
Initially, the test questions are shown to the reader as gigantic, terrifying monsters. Then, as the students of Class E remember the lessons that Koro-Sensei taught them, they transform into something weak and harmless. Up until they hit the questions that the Chairman had put in intentionally to trip up class E, they were doing extremely well. Had it not been for the Chairman's interference, they might very well have all reached Class A! Whatever you might say about Koro-sensei's unique methods or his unusual appearance, he is an EXCEPTIONAL teacher.
Then Karma shows him he got almost perfect marks in all of his exams - and tells Koro-sensei he wants to stay with Class E because he likes the teacher's method and the class.
Terasaka atoning for his mistakes by using the money he got from Shiro to buy tasers for the group, fighting to help save the sick students even though he's suffering the same illness as they are and being the one to tell Nagisa to Get A Hold Of Yourself Man.
The students trying to help Irina hook up with Karasuma.
Terasaka talking Itona down from his rage rampage. The bully has come a long way indeed!
Chapter 90 (episode 5.2). Gakushuu challenges Class E to a strength contest because he discovered Isogai's part-time job. When Isogai tells the others not to worry because he's gonna take full responsibility for it, the rest of the class, who have done nothing but praise and admire him in the first part of the chapter, call him an idiotfor thinking he would be alone.
Chapter 91 (same episode). Koro-sensei being a doting parent for his students, completed with him taking pictures of everyone, then helped Isogai to put on his bandanna while encouraging him.
Chapter 95 (episode 6.2). Despite Koro-sensei slapping the entire class for injuring an old man, the next chapter reveals that he apologized to all the student who were uninvolved in an incident at mach speed beforehand. He still punished the whole class, believing that it would be unfair unless everyone were treated equally. And Koro-sensei did all this while being absolutely furious.
Fridge Heartwarming comes into play when you realize that Karasuma could've reported Korosensei for his actions (and Koro-sensei was fully aware of it), yet he chose not to.
Nagisa managing to connect with Sakura, helping her study and get better marks at school.
Chapter 110 (episode 8.2). After brutally turning down Irina's affections (leading to her basically betraying the entire class), Karasuma sort-of apologizes for being a dick and tries to make amends by giving her a rose.
Chapter 114 (episode 9.2): Despite his mother attempting to burn down Class E to force Nagisa into the school proper, and having been controlling his life to allow her to relive her own, Nagisa doesn't burn his bridges with her. He still loves her as a mother, is grateful for all the good she actually has done for him and tries to persuade her to allow him to live his own life and takes some more responsibilities on himself to allow her to relax a bit more. Though she seems begrudging about it, she accepts this.
Chapter 116 reveals that Nagisa still visits and tutors the kids in the daycare center, and becomes really popular with Sakura and the other kids there.
Chapter 117 (episode 10.2): Yuuji returns, having fallen in love with Nagisa, but is crushed to learn he's a guy. Even after that, though, he still writes a glowing review for class E's shop, which gives them a chance to compete with class A thanks to word of mouth advertising.
While having lunch together, Yuuji tells Nagisa that he's cleaned up his life and making something out of it. While it is eclipsed by the Cringe Comedy of Nagisa trying to hide his true gender, it shows that he is already making a positive mark in the world.
Past assassins stop by Class E's shop, generally bearing gifts, including Irina's mentor. Karasuma gruffly tells him to go see Irina, as she had been worried.
Chapter 118 (same episode as the previous one): After all of the Nightmare Fuel she provided just a few chapters ago, Nagisa's mom comes to realize how happy her son is with Class E, and decides to stop treating him like an extension of herself.
As a background event in one panel, we see Maehara and Kataoka extending an olive branch to Kaho Tsuchiya and Kokona Tagawa (the former being Maehara's manipulative ex-girlfriend while the latter was Kataoka's guilt-tripping ex-friend) by serving them food, which both girls seem to be enjoying.
They're sick, they're twisted, and frankly downright evil (Gakuhou even more so than Gakushu, but still), but when you get right down to it, there's just something downright family-like about Asano Sr. and Jr. threatening to sue one another for millions of yen on the ride home from yet another failed attempt to destroy Class E.
In the anime (episode 13.2), something a little different happens. After losing the handbook-grenade game, Asano tells his son that suing him for domestic assault will never work because from then on, he'll continue to improve himself as an educator... and as a parent. Aww!
In the previous episode: It's the last problem of the Math exam. While Gakushu is using brute force to come up with the answer, Karma takes some time to reflect on his time in Class E and it helps him solve the problem. It's nice to see the personal growth from the jerk who acted like he didn't need anyone else in his introduction into someone who understood that his time in Class E had aided him on his journey. The music is particularly touching as well.
Chapter 126 (also episode 13.2) defines Class 3-E's relationship as a whole to Koro-sensei. Because even if the Class E building gets demolished and Koro-sensei gets the boot out of Kunugigaoka, his students are willing to follow him to the ends of the earth to complete their education and kill their teacher. Even if they have to ditch school, and run away from home in the process. It is later revealed that Koro-sensei modeled his entire teaching style off off of none other than the principal back during his teaching days which he lost faith in after one of his students committed suicide after being bullied.
An unexpected one in chapter 127 (end of that same episode). When Isogai tries to complain to the student council about the problems of the school play, Asano gives his usual crap, but adds:
In chapter 132 (first half of episode 15.2), even though she has just tried to kill him in a fit of tentacles-induced madness, Nagisa tenderly supports Kayano's head as she slumps to the ground. In the next chapter, Okuda also comes forward to hold her.
Chapters 134 onwards (episodes 15.2 and 16.2) - the flashback arc discussing Koro-sensei's origins:
Though the ending is a given tragedy, it is absolutely adorable to see the God of Death and Aguri form a relationship. He initially notes her as a point of manipulation, then is ready to discard her as a pawn when he realizes she has no real value. Then he is defending her from her asshole fiance and giving her pointers on lesson plans. As the experiment on him starts taking a toll on him (at one point, he's spitting blood all over his room), she is the only one who shows him any modicum of compassion.
In Chapter 137 (episode 16.2), Aguri laments how even though he was the only one to give her support, she cannot touch him. In response, he forms a hand made from ultra-thin tentacles that gently touches her cheek. Seriously, look how adorable thisis.
"It's all right. You can do it."
Koro-sensei's dedication to the promise that he made with Aguri before she died in his arms is this and a Tear Jerker. It shows just how much her compassion has changed his view on life. Even though Koro-sensei could do anything with his new powers, he chooses to continue teaching in her place and genuinely wants to change Class E's lives with the little time he has left.
And before that, it's thanks to her that he knows the difference between looking at someone and looking at someone and ensures to apply it to the students she left behind.
Koro-sensei's big, silly necktie is a Tragic Keepsake, as it was the gift that Aguri got for him. Considering how he felt about her, it's doubly meaningful; previous chapters establish that it covers the location of his heart.
While kissing Kayano, Nagisa thinks that he refuses to believe what she said about everything being a lie.
Chapter 142 (the very end of 16.2): Nagisa tells the rest of Class E that instead of continuing to try and kill Koro-sensei, he wants to look for a way to save Koro-sensei's life.
Chapter 143 (17.2): The class becomes split into sides that want to try to save Koro-sensei or kill him. Koro-sensei intervenes and proposes that the class should decide what to do with a paintball game; he doesn't care what his students do, so long as they do it together.
"The thing I would hate the most is that everything ends with the class divided into two... if you are doing this for my sake as well... then at least promise you will be united until the end."
Fridge Heartwarming here, but the reason why Koro-sensei was so pissed by Itona's tentacles? It's because he realized Itona went through the same torture he did when he was a test subject. He wasn't mad at Itona, but at Shiro! Itona may have been an Ax-CrazyCreepy Child, but he was Koro-sensei's student all the same. And when it became clear that someone had hurt a student of his so deeply...
Chapter 144 (17.2). No matter what their side, every student's given reason for joining their respective team is one. A notable one is Kayano's, who joins the Blue team to protect her teacher and the man her sister loved despite wanting to kill Koro-sensei just a few chapters prior.
Chapter 146 (episode 18.2): At the end of the fight between Nagisa and Karma, once Nagisa gains the upper hand on Karma, as the two are exhausted from fighting, it almost looks like the two are hugging it out.
Chapter 151 (episode 19.2): "But to us, your life...is more than just teaching material."
The ISS astronauts do not get angry with Nagisa and Karma for the bomb threat (not even when the bomb is just jelly and the knives are fake), and actually tell them to return the legal way the next time.
Also, Ritsu interfacing with the craft as she steers Nagisa and Karma back to Earth. She says she finally knows what it's like to feel. And we can tell from the tone of her voice.
Chapter 159: Kayano :gave Koro-sensei a picture of her sister for Valentine's day, stating she would want him to have it.
The real reason Karma had her give that photo: So that she can hand her valentines chocolate to Nagisa without Koro-sensei interfering or getting any dirt on it.
Chapter 160: After a Valentine's Day dinner, Karasuma and Irina are discussing the possible futures of Class E. Irina brings up how she's genuinely worried for the students' well being and that she doesn't want them to have a future tainted by blood. For all her violent tendencies and irritation toward the class, it's amazing to see how much they've influenced her.
Karasuma suggests to Irina to quit becoming a hitman after their business with Class E is done, and instead to join him at the Ministry of Defense so that way she can use her skills properly. She can also atone for her sins by dropping by a shrine and praying for the souls she killed, but Irina points out that she's Christian and that she doesn't even have relatives in Japan. Karasuma quickly corrects her by pointing out that she can't pray at a church because there are no churches near his house.
Chapter 165: Koro-sensei is trapped in a situation it looks like he can't escape from. What does he do? Spend his time inking memories of the time he spent with Class E.
Chapter 169: Koro-sensei being so proud of his students after they've beaten the mercenaries and their boss. They also try to playfully kill him, but you can see how happy everyone is.
Chapter 170: As Koro-sensei calmly accepts his death by Kill Sat. Class E reveals that they got him a cake. It's one year since the moon explosion, which means it's one year since Aguri gave him a birthday. However, Yanigisawa and the God of Death show up.
Chapter 173: After Koro-sensei repeatedly takes hits from the Reaper 2.0 meant for his students, Yanagisawa gloats at him and states that his students are his biggest weakness. Koro-sensei retorts with a Shut Up, Hannibal!:
Koro-sensei: THAT IS PREPOSTEROUS! You're a fool if you think I'll believe it's that cut and dry! These children, they've risked life and limb to save me! What they've gone through just to be here tonight is extraordinary! Their determination... their unflappable spirit... students like these are the greatest gift a teacher could ask for! They are not a hindrance! They are not a weakness! They are my class, and I am proud of each and every one of them! Call it what you will, but when the chips are down, I will give my life to protect them!
Chapter 175: Koro-sensei reveals his ability to essentially bring somebody back from the dead, which involves completely rebuilding their body at the cellular level. Koro-sensei reveals that he had been secretly developing this ability ever since Aguri's death, since he never wanted to see a loved one die in front of him again. Especially if he had the power to save them. The fact that he does this to save Aguri's sister makes it all the sweeter.
Chapter 178: Pretty much the entire chapter.
It's slightly Played for Laughs, but Koro-sensei temporarily freaks out about being assassinated without having a roll call. It does break from the dramatic moment his death provides, but it does go to show that he loves the class enough that he would want all of them to be present for what is about to transpire.
After killing Koro-sensei, Class E returns to the classroom to find massive Doorstopper graduation albums and a personalized consultation guide for each and every student, all literally handmade by Koro-sensei himself.
For the first time all year, Class E isn't mocked at the student assembly.
Karasuma keeping the Army at bay so that Class E could have some private time after killing Koro-sensei. Karasuma dares them to try anything in his presence. They don't.
Koro-sensei convincing Nagisa's parents to get back together so that Nagisa could have the chance of living with a real, healthy family.
The Chairman privately congratulating Nagisa at the graduation ceremony, and actually complimenting Koro-sensei, showing how much respect the Chairman has for him.
When the media tries to crash the graduation ceremony to interview Class E, Class A steps up and helps Class E escape, stating that despite their differences, they're all still students at the same school and it's their duty to look out for each other. Aside from some banter between Karma and Asano, the Big Five themselves happily see them off with no hard feelings.
Chapter 179 has several more moments.
Asano ends up having to close down the building for class E while also being fired for the students being put in danger. As he's packing his things, his old students arrive all grown up to help him pack and celebrate his career. Even Asano's regular stoicism turns to genuine happiness over seeing his old pupils.
Cutting over to Karasuma and Irina after the 7 year timeskip, we see Karasuma sending her on a mission and find out they ended up married.
Finally, we get the all grown up class E returning to the old building to repair it as a monument to their old teacher.
Chapter 180, being the final chapter of the series, shows that, everything is coming good and dandy for the Class E's alumni.
Especially Nagisa, who gets threatened by his deliquent students in his first day as a teacher... and he effortlessly shows them who is the boss. Then, he turns the offending pupil around and straight up tells him that he'll look forward to seeing if he can "kill" him before graduating. And Nagisa's killing intent, which was always represented by a viper, is instead represented by the image of Koro-sensei. "Class, be seated..."
comes forward to hold her.
