In Episode 9, Sei and Reiji train China for the tournament all done to the theme song.
The sheer amount of effort that China will do for the sake of Sei is heartwarming on it's own.
Episode 10, China gave Sei and Reiji Haro-shaped lucky charms. Reiji recognized it's just an afterthought but still happily received it while Sei visibly blushes.
In Episode 11, Reiji randomly runs into Aila again. After taking a meat bun from her bag of snacks (calling it "payback for last time"), he runs off... but then throws her some money. After a stunned moment, Aila realizes that he gave her enough money to buy four meat buns.
Episode 12, Mao and Fellini's refusal to leave Sei and Reiji to fight the Mega Sized Zaku alone. This shows that even though they are rivals, they are still True Companions.
Aila buys meat buns for Reiji with the extra money he gave her and waits for him to show up for an unknown amount of time in order to personally deliver them to him. She even insists that Reiji take them when he refuses (on account of them being cold) because she doesn't want to feel indebted to him.
Minutes later, Reiji stands up to a group of older teens to defend Aila after she insults them in response to their teasing. Even after one of them pulls out a baseball bat, Reiji refuses to back down. The Stinger reveals that Reiji had his arm injured as a result of the fight but Aila looks to be unharmed, meaning that Reiji protected her at his own expense.
Episode 13 shows that, indeed, Reiji tanked a full-strength swing from a metal bat on his arm with little initial discomfort, then drove all three off.
Aila finding and thanking Reiji the next day.
Episode 14 gives us a new ending, the lyrics of which describe remaining true to the things you love and enjoy, no matter how much time goes on. The perfect song to describe the Gunpla loving Man-Children who fill the show, and the fans who've loved Gundam just as long.
Episode 15 gives us a flashback that shows Ricardo Fellini as a child, playing with a custom-colored Wing Gundam model. As he grew into a man and became a champion fighter, the Wing "grew" with him, being gradually modified into the Wing Fenice as it is today. The two of them have literally been together for years; is it any wonder why Fellini refers to his Gundam as "Partner"?
On a meta level, it's almost as if the lights coming on are the Fenice's way of saying "You got it my friend. We're together to the end of the line!"
Another moment within that montage itself: After the scene from his childhood, you cut to him as a teenager, using the Gunpla Battle system. The huge smile on his face makes it clear that this is the very first time he saw his favorite childhood toy, that he's kept with him all that time, come to life.
When the match ends and the announcer declares the match is a draw, the audience slowly clapping before going into full on applause is enough to bring a tear to one's eyes. It certainly did for Kirara and China.
The episode ends with one final heartwarming scene: Sei and Ricardo are rebuilding their Gunplas together. Friendly Rivalry at its finest.
At first it's not really certain whether Kirara/Mihoshi really likes Fellini or not, but as the tournament progresses you actually see her getting concerned whenever Wing Gundam Fenice is in danger. And that she's absolutely pissed that Rainer was demeaning Fellini.
Despite its silliness, Sei's dream in the beginning at the beginning of Episode 16 really shows just how much he admires his father. And at the end, Takeshi quietly apologizes for not stopping by to visit Sei at the tournament but states that he has been watching his matches on television. The pride in his voice is apparent.
The Stinger reveals that Takeshi actually came back to the hobby shop with the intention of surprising his wife. Too bad the store was closed.
Episode 20. Reiji charging in with his Beginning Gundam to defend Fenice from Quebeley Papillon—not only because of his respect for Fellini, but now that Reiji has built his own Gunpla, he know understands how close Fenice is to Fellini. Yes, he got his Beginning chopped up, but at least he managed to prevent further damage to the Fenice.
On that end, Kirara intervening to prevent Fellini from self-destructing the Fenice. Yes this is a troubling subject for certain people, but think about it in this context: Fellini was about to sacrifice his most treasured Gunpla, one that had been at his side since childhood, in a random battle that would have gained him a Pyrrhic Victory at best (as well as left him unprepared for his next showdown with Sei and Reiji), and Kirara, knowing how much the Fenice meant to Fellini, stepped in to remind him of its value and that this one fight was not worth its sacrifice. That says a lot considering how selfish and uncaring Kirara was at the beginning of the series, and much more how her apparent relationship with Fellini has progressed.
Episode 21: Fellini, despite the severe damage to his Wing Gundam Fenice, tries to get Reiji to let go of his anger at Aila. Even if he has to take a punch to do it.
Sei figures out how to patch things up between Reiji and Aila: he has China give her the stone Reiji gave him back in Episode 1. And just like for him, it connected them together.
Reiji finally gets to talk it out with Alia. After learning the truth about her, he promises her a place in Sei's house, giving her a chance to escape Team Nemesis. She accepts and removes the Embody System so she can fight a Gundam Battle the way it was meant to be fought: for fun.
Episode 22: Despite her earlier reservations, Rinko immediately offers Aila a place at the Iori household until she is able to support herself after learning of her background.
Yuuki rejecting Meijin II's philosophy of victory at all costs, reaffirming his belief in friendly rivalry, and reestablishing his relationship with Julian.
Episode 23: All the most tragically ending relationships in the Gundam Saga thus far having their happy endings in another world, another time.
Uso Ewin, the 13 year old main character of Victory Gundam—one of the most tragic and violent Gundam series ever—gets one scene where he finally gets to be just an ordinary kid, playing toys with his parents.
Outside the tear-jerking meta context, the show pulls off a few moments for itself too, like China helping Alia build Miss Sazabi, Yuuki getting to be himself for a while, showing just how far Sei has managed to improve thanks to Reiji, and finally reuniting the Iori family, with a little help from Mr. Ral and Char Aznable, of all people.
Hell, Gundam Build Fighters Episode 23 is THE Crowning Moment of Heartwarming for the entire Gundam Saga. At least until the plot kicks back in.
Just how everyone's bro-ing it up, even rivals. The episode's pretty much captured the feeling of being at a good con where people are friendly and all into the same things so it's OK to be a huge nerd.
Aila being so enthusiastic about building her own Gunpla, especially considering what she went through the previous few episodes, it's nice to see that without the meddling of the Flana Institute, she is finally able to enjoy a hobby with friends.
Episode 24 shows that, unlike what some fans may have feared, Allan is very much NOT okay with Yuuki being forced to use the Embody System. He has to be physically restrained but still manages to find an opportunity to break free and fight the PPSE goons for the system's control.
Episode 25 is full up of Heartwarming Moments, but the stand-outs would be both Baker and Aila following Mashita and Reiji back to Arian.
At the very start of the episode, Nils tells the gang his plan, and urges them to evacuate. Cue everyone—Reiji, Sei, China, Aila, Fellini, Kirara and Caroline, telling Nils they aren't going anywhere, they're all going to take down the crystal together.
The immediate first thing Takeshi does is to start evacuating the viewers, aided by Rinko, Chinnan, Ral, and other Gunpla fighters.
This also overlaps with awesome: The reason for the Big Damn Heroes by the old guys? Because they were worried about their kids. When Rinko told Takeshi Sei and Reiji were missing, and Takeshi realized that they were still inside the stadium, the smart thing would have been to leave that to the pros. But fuck that noise, his son is in danger, he won't stay back.
Also, Ral and Chinan didn't need to follow Takeshi in, but they did so anyway. Call it the bond of the battlefield, call it concern for the kids, call it basic human decency. Doesn't matter why they followed him in, just that they did.
When they're storming A Baoa Qu, a giant beam cannon fires. China and Aila sacrifice their Gunplas to push Sei's and Reiji's out of the line of fire. And then the boys stick around to protect them, refusing to leave themů until Yuuki in the Amazing Exia Repair and Takeshi in the Perfect Gundam show up to protect the girls, allowing the boys to destroy the crystal.
In order to facilitate the promised final battle between Sei and Reiji and Yuuki, Mao and Fellini donate parts of their own Gunpla to help Sei repair the damaged Star Build Strike back to fighting condition.
As Sei, Reiji and Yuuki prepare for what might be the last Gunpla Battle, Kirara grabs a cam.
Kirara: "All the Gunpla fans around the world, are you watching this? If you are, keep your eyes on this fight!"
When Reiji, during the last duel with Yuuki, realizes that he's about to be sent back to Arian, what does he do? He lets Sei take the commands and encourages him to realize he's a great pilot. Reiji uses his last moments with his friend to give him a wonderful gift.
When Sei, during the duel, scores the victory attack, Reiji's and Takeshi's faces just screamSo Proud of You.
Aila uses her Arista to follow Reiji to Arian, because she loves him. Bringing his Beginning (which he'd forgotten) pretty much sealed the deal.
The ending credits shows what the main cast (and other characters) has been up to after the World Tournament. Highlights include:
Kirara fulfilling her dream of becoming an idol and performing at the Budokan, with Fellini in the audience watching over her. Also a callback to her concert in Episode 23.
Allan and Tatsuya promoting Gunpla together.
Mashita and Baker selling their mock-Gunpla on Arian.
Nils and Caroline recreating the Plavsky Particles and saving Gunpla Battle.
A leaked image from the Blu-Rays shows an older China mailing a letter to her family from Paris. Standing guard over her desk are Mr. Bearguy III... and Aila's Command Gundam.
After Season 1's final episode aired, 4chan's /m/ message board launched "Gunpla Party." Hundreds of photos of Gunpla mimicking Star Build Strike's victory pose in Episode 24 were collected and sorted into a collage◊, which was sent to Sunrise staff with a message of thanks and appreciation for Build Fighters.