Kikaijima and her senpai value money more than their lives, but they also value each other more than money.
After getting up from being turned into a human pincushion in his fight against Munakata Kei, Zenkichi is visibly pissed, not because of the five blades sticking out of his back, but because Munakata dared to make Medaka cry.
Definitely understated, but Munakata's backstory. Munakata's killing impulse is the real deal, so he had himself branded as a wanted criminal and purposefully isolated himself from others in order to avoid killing anyone. His main reason for joining the Flask Plan was to find people strong enough that they could survive being friends with him.
And of course, Zenkichi is badass enough to do just that.
Zenkichi manages to do something similar in later chapters, being able to convince Emukae Mukae that just because she was a Minus doesn't mean she couldn't be accepted.
And of course we can't forget how Zenkichi pretty much persuaded Medaka that it was her purpose in life to make the people around her happy.
The whole fight between Medaka II and Zenkichi, can also be considered a crowning moment of awesome for Zenkichi, and the last nail that made the relationship blatant and obvious for everyone in-universe.
Doubled with a flashback that shows what is Zenkichi to Medaka.
Even though the same one makes you wonder how such a sweet kid ended up being Zenkichi.
And of course the fact that Medaka made Zenkichi's words her life motto TWICE. And then again by how Zenkichi admits that he doesn't remember the events of 13 years ago (who would blame him) but has never changed how he thinks of Medaka from that day forwards.
Zenkichi accepting Shiranui and declaring her his friend, even though she'd just made herself his enemy and seemed to be trying her hardest to alienate him.
Medaka's fight with Kumagawa ultimately culminates in one that overlaps with an Awesome Moment:
Medaka: "Crawl back up, Kumagawa. I look forward to the day that I get to save you!"
In the same chapter Kumagawa apologizes to Class Minus 13 for losing, and declares that it wouldn't matter since he would willingly sacrifice himself to protect the Minuses.
And then we have Class Minus 13 saying Kumagawa was already doing that, that it was now their turn to protect him, and promising to improve their ways once they saw that even someone like Kumagawa could be reformed.
It's also a heartwarming moment for Kumagawa; his defeat by Medaka is the first time that he's lost properly. Not because of fate, not because of his Minus, but because of only himself. It was the kind of defeat that helped people get stronger afterwards, giving him the hope that someday, he'll win.
We also have Medaka telling Kumagawa that she needed him in the Student Council.
Takarabe refusing to apologize to Aka, because Aka bullied her friends. Despite being scared to death.
Kumagawa winning against Aka just to get her to apologize to Takarabe and her friends for looking down on them because they didn't have an ability (and maybe get the nurse to go Naked Apron).
In chapter 119, Kumagawa announces that he's willing to support Kikaijima in stopping Anshin'in's plans.
Munakata willing to break his code in order to help his friends, despite being somewhat aware that his intentions won't justify his actions.
Of course, Kumagawa protecting Kikaijima from Munakata, claiming that he feels happy dying for a girl.
And Medaka's successors opting to follow Kumagawa, saying that in order to return things to normal they're willing to make as many enemies as needed, including Anshin'in herself.
In a meta-example, there's something inexplicably heartwarming about Kumagawa's total and complete victory in the second character poll. Seeing him cry tears of happiness at finally winning for once (and, one assumes, for other things) is rather touching.
What truly makes it heartwarming is that this is a character who has never fairly won anything in his life. Ever. And has never had anyone really like him for who he is who wasn't a minus. Ever. And he won a popularity contest. Against the main characters of the series. And he didn't just win, he dominated to the point that he got more votes than the 2nd, 3rd, and 4th runners up. COMBINED. When Kumagawa declared his true intentions, he ended it with saying "I want to prove that I'm better than the main characters!" You are, Kumagawa. You are.
And then he did it again with the third character poll, with Akuna and Zenkichi coming in 2nd and 3rd respectively.
Akune coming to terms with his feelings for Medaka, allowing himself to finally be able to move on and go all out.
Though there is an undercurrent of manipulation from Ajimu, it's definitely heartwarming to see how many people are willing to support Zenkichi in Chapter 122. Maguro, Naze, Shiranui, Emukae, Munakata, and eventually Akune.
In chapter 126, Emukae sets aside her own feelings and declares that Zenkichi's "selfish" love for Medaka isn't wrong, because it's love.
Shiranui: You see, unlike justice, love does not always have to win.
Medaka mentioning that, when she looks back on her high school life in the future, she'll be proud to have been the person Akune loved.
Chapter 139: Zenkichi telling Medaka that she has to find her own reason for living, then telling her that her speech about "living for strangers" is wrong, and that no one at the school thinks of her as a stranger any more.
Chapter 168: Najimi and Kumagawa's final conversation, when she hugs him and tells him he is strong, a symbol of hope for people who've lived in despair, and that she loves him.
In the same chapter, when she blew up a star for him.
To go into more detail, she says "So what, you were born under a loser star? Was it this one? Or maybe this one? Well, let's just say it's this one," and then shoots the star out of the sky with a Finger Gun. In effect, she's encouraging him not to let the idea of being a Minus hold him back, and to go for victory and happiness.
Chapter 176: Zenkichi forgives Shiranui for her Jerkass Fašade after all the trouble they've gone through.
Chapter 184: Iihiko's "echo" comforting Medaka as she tries to psych herself up for a course of action that may likely result in her death. For just a moment, we can see why this guy was considered a hero.
When Medaka finally returns. Kumagawa can be seen crying. He truly missed her
Just as heartwarming is the words they exchanged when she returns. He bet her that Medaka would return in time for the graduation ceremony and Medaka admitted she lost. Kuamagawa tears up when he declares "At last, I won!".
Chapter 190: Kumagawa reveals that on the back of each flower of the bouquet's nameplate is a message from its owner. It's overwhelming seeing how much people are going to miss Medaka.
Actually, the whole finale of the manga is being one huge Crowning Moment of Heartwarming, with the battle with all the characters of the manga in a run, bar Anshin'in, the end of chapter 190, and the Where Are They Now moment in the final two chapters. A satisfying ending to a manga that was certainly different from Shonen Jump standards.
In chapter 192, a 'ten years later' chapter, Zenkichi challenges Medaka to a final duel to settle things. If he wins, she marries him. She accepts, conditioning that, if she wins, he has to marry her, promising that they won't ever say goodbye ever again.
Earlier, Kikaijima asks Zenkichi to hold her newborn son because she wants him to grow up with the same strong willpower as Zenkichi.
In a side arc in chapter 192, Saki sings a sing to Kumagawa encouraging him to keep trying to succeed in his goals. He later leaves her a banquet of bolts.