The relationship between the Ha-Ha Brothers is this in its entirety, but of special note is this: Juumonji's father tells him that shouldn't associate with 'trash' like Kuroki and Togano. On the same day, Juumonji sees himself being praised in a sports magazine (in an article written by Kurabokuro) mentioning the Devils, while his two friends are once again dismissed and insulted. He then tells Kuroki and Togano that the magazine said all three of them were trash, and the two spend the month up until their game against the Americans seriously training in an effort to prove their worth. During said game, the linebackers help Sena to score in a sweep attack, and when the announcers begin to praise Eyeshield 21 once again, Kurabokuro apologizes to Kuroki and Togano and leads a cheer for them. That moment nearly brought this troper to tears.
Expanded in the anime where Kuroki revealed he read the magazine and knew that Juumonji was covering them. He and Togano decided to leave the death march even without Juumonji because by saying that they are trash. Juumonji then punched Kuroki saying that he will pick a fight with everyone who dare call his friends trash, including them. After beating each other they decided to reunite each other and finished the Death March.
For another, when Leonard Apollo, coach of the NASA Aliens, finally realized that he had been treating Panther the exact same way his coach in the NFL treated him, saw that Panther had always respected him and been a fan of his despite the abuse he piled on him, and gave Panther the spot on the team he always deserved.
Or when all of his team knelt down just so that Panther may enter the game.
Even moreso during Panther's first play, where he takes the ball, dashes right through Kurita and Komusubi and outruns Sena for a touchdown. It's like all his dreams have come true at once.
Panther: Ever since elementary school... right, Grandma? Here I am, wearing this uniform and running on the field. (tears in his eyes as he runs) I can do it! I can play football!!
Then there's Daisuke Atsumi of the Yuuhi Guts during the last moments of the game against Deimon. "I'm Daisuke Atsumi... and this is my last play!" The fact that the Guts were going to lose no matter what, and Atsumi's last play was destroyed by Eyeshield 21 himself kind of compounds it.
Sakuraba gets a lot of these. When his fangirls come back to him, for starters.
When he heard Takami's praise and hopes for him from behind the wall.
Monta receiving a compliment from his hero, baseball player Masano Honjou after catching a ball aimed at the back of his head.
When enormous Hakushyuu Dinosaurs lineman Rikiya Gaou threatens to rampage through the stands breaking bones to avenge an insult from the audience, Mamori immediately pulls Suzuna behind her to protect her, but then Hiruma leaps in front of them, brandishing multiple tazers, and immediately afterward the Ha-Ha Brothers all jump up wielding their delinquent bats to follow Hiruma's example.
The scene at the end of the Bando Spiders game when an exhausted Sena staggers in and collapses into Suzuna's arms. Her initial shock and embarrasment gives way to acceptance and she asks him if he's grown taller.
Hiruma forgetting about facts and numbers for one second and actually straight-up believing in his team. And the pay off as him screaming one of his loudest most happy Ya-Ha.
Musashi watching the Devil Bats losing on the television in his father's hospital room, unwilling to return to the team waiting for him until his father punches him and the workers at his company all show up to tell him that they'll carry his share of the load.
When Monta gets discouraged about having to face his idol's son (and because Sena accidentally wrecked his prized catching glove), Sena runs around collecting the gloves of the other receivers and cornerbacks he faced to rally his spirit and confidence.
When Yukimitsu isn't picked to start for the Devil Bats and breaks down in tears after everyone leaves. Sena and Monta, knowing it will just make it worse if they go and comfort him, loudly declare off to the side that they will stay in the tournament as long as they can so everyone (meaning Yukimitsu) can play.
After Deimon beat Ojou in the winter tournament, we get to see Kurita and Ootawara hugging each other and crying, each happy and sad for the other.
Before the Christmas Bowl, the Teikoku Alexanders with their huge roster, seemingly invincible record, and solid tradition of winning even have a huge snow sculpture of their mascot at the entrance of the Tokyo Stadium. Habashira Rui (of all people) sees this and gets pissed off that one team should have so many advantages while the other team has so few and orders his team/gang to the Tokyo Stadium to build an equally huge snow sculpture of the Devil Bat. Awwwww.
When Hiruma realizes it's impossible for them to win the Christmas Bowl he takes a moment, puts on his scary face and tells his team a whopping lie about their great chances. They all know he's lying. They believe him anyway.
Hiruma gets one when Zokugaku loses their chance at the Christmas Bowl and Habashira Rui comes to him for advice. "YOU AND ME, WHAT'S SO DIFFERENT!?" Hiruma gives no answer, but the panels to his left clearly show what he thinks.
Hiruma after breaking his arm facing down Gaou and lying on a stretcher kicking a crying Kurita in the butt to get him going.
Or, when Agon realizes he's going to be crushed by Don, and there's no reason why Kurita would even think about blocking for him, after such a jerk Agon has been. And then Kurita blocks Don anyway.
Musashi's triumphant return to the Devil Bats: "You're thirteen thousand, two hundred and ninety-seven hours, forty-nine minutes late."
When the Devil Bats have tryouts, the candidates are to climb Tokyo Tower carrying a bag of ice in extreme heat (along with other obstacles). If they make it to the top with at least one ice cube left, they make the cut! As you can imagine, mostly everyone gives up. But not Yukimitsu even though he's clearly no athlete. He finally makes it to the top, but all of his ice has melted. Hiruma then sneakily grabs an ice cube and plops it into the bucket, informing everyone that Yukimitsu has qualified for the team!
To elaborate, Kurita has just met Hiruma, and wants to make a Football team no matter what. Hiruma refuses, saying he has somewhere to be. Kurita follow him to an Army Base, where he sees Hiruma enter through a hole in a fence. Kurita then sees Musashi, a carpenter, working to fix the hole. Kurita begs Musashi not to, since that means Hiruma won't be able to come back anymore. Musashi still fixes the fence, but kicks another hole through it, saying there's no problem.
The Rousing Speech Hiruma gives to Musashi right before the final play of the Christmas Bowl. Sorry, but it must be quoted; "When you left the team that day, do you know how much Kurita hit things in despair? And when he ran out of things to hit, do you know how much he screamed? Kurita... Do you know how much Kurita was...!" (background panel flashes to Hiruma himself kicking the bench out of despair) "KEKEKE! YOU'D BETTER SETTLE THAT DEBT RIGHT HERE, ONCE AND FOR ALL! SCORING THAT IMPOSSIBLE KICK IS YOUR JOB, FUCKING OLD MAN!"
Marco, post-defeat, realising that in trying to win over his crush, Maria, he's driven her away completely...and in admitting it, winning her over again. Ahhhh.
Sena's father cheering Sena on and telling him to continue on, even before knowing that his son was Eyeshield 21.
During the World Cup, we learn the extent that Habashira Rui works, in Hiruma's words, "for the sake of guys who don't even care about their future." It puts an excellent spin on why he acts like an asshole but still does decent things—he's trying to control Zokuto through fear, not just for his own sake, but to help delinquents who had already given up on themselves. Hiruma himself says that Habashira has always had to bear the burden on his own, unlike frankly everyone else in the series.
At the end of the World Cup, the teams tied, and Morgan declares the game over. When neither teams want it to end like this, they decide to go into overtime. However, instead of seriously playing to crush their opponents by putting all the starters up, Team Japan puts Sena at quarterback. The fans are gone, the tournament is over, and they're just guys playing football and having fun. Anyone who's ever played football will understand how amazing this feeling is.
When Kurita loses to Gaou in strength, and Hiruma's arm is snapped, he loses the ability to understand Komusubi, meaning he is no longer a strong man. Komusubi then, for the first time in the entire series, speaks in a complete sentence to get his master motivated.
The sheer amount of Character Development that everyone goes through in the Distant Finale. Kurita has gone from thinking, "I don't know what I would do without Hiruma," to "Should I follow Hiruma, and leave Musashi on the side? Never. It's best we go our separate ways."
Hiruma vs Sena. Also part of the Distant Finale. While we never get to see it played out in full, Hiruma vowing to beat one of his closest friends, and Sena looking forward to the tricks Hiruma will throw at him, made this scene perfect.
Kid and Tetsuma. The reason that Kid picked up American Football... was so he could play with Tetsuma. Moreover, Kid decided to aim for the Christmas Bowl because Tetsuma wanted to face Monta again. Of course he was then destroyed by Gaoh immediately after. We then see one of the few displays of emotion from Tetsuma in the whole series, he's so furious that he suicidally charges Gaoh as a block for Riku. You really see how much these two care about one another.
A small one during the NASA game when the younger Gonzales brother is revealed. In flashback, he wanted to play alongside his big brother, but other people mocked him for being too much of a pipsqueak to play football. But his brother just put a comforting hand on his head and encouraged him to ignore them and play.
Elder Gonzales: You have your own skills that make you special. Don't worry what other people say- jump right on in!
Gaou speech about being the Determinator during the World Cup, it is essentially him praising Marco for all the sacrifices he has done in order to pursue his dream, Marco never thought Gaou actually cared about him is surprisingly shocked.
The Poseidon's quarterback lampshading how useless he is to the team and that the line and Kakei is the reason they do so well. None of his teammates, even Mizumachi who is ridiculously tactless, agrees. And it's not to spare his feelings, they don't see why he would even say that.
The story of the Bando Spiders in itself is a case of multiple heartwarming moments.
First, there's Kotaro who takes his position as the kicker very seriously, and despite his unit always being sidelined in favor of the offensive and defensive teams (and being mocked for it too), always practiced to prove that kicking in American Football is as important as the offense and defense. When both offensive and defensive teams were recruited to Teikoku, he alone rallied the kick team and even turned Bando into a credible team.
Then there's Akaba, ace of the Bando Spiders. He and Kotaro annoy each other to no end, and would always bicker over the smallest things, but deep down, they respected each other. The reason he left Teikoku was that despite being in a team where he could've succeeded and excelled, he found that there was something lacking: that nobody was actually enthusiastic in playing football, even his former Bando team mates, who only wanted to win. He remembered Kotaro, the guy who shared his passion for football and decided it was more fun playing with that guy. Thus when he returned to Bando, he vowed to make the spiders the ultimate kick team.
And Akaba's request to join team Japan? Have a kick team with either Musashi or Kotaro to show the world the importance of such a team.