You can see the exact moment Gyanko fell for Sekai when he praised the shield her brother gave her.
Episode 5
Fumina tackle hugging Sekai and Yuuma after their first win. You can clearly see how happy she is to finally make it out of the first round of the regional tournament after having failed so many times in the past, and is thankful for her friends who helped her achieve that.
Episode 7
Shimon's story in general. To keep his sick little brother's spirit's up, he takes his bro's simple-assembly gunpla and whoops the tar outta everybody. He even drops the chance to defend his title as Japan's boxing champion in order to participate in the Gunpla tournament.
Gorou and Kouji. They may be shit-tier fighters who're only being carried by Shimon's skill... but they're true bros, going as far as to beg Team Try Fighters to let Shimon win, and pulling off a Heroic Sacrifice each to even the odds for Shimon.
Or in the end, Shimon grows to love Gunpla battling after a great battle with Sekai. Even promising to use his little brother's gunpla next year.
How does Sekai resolve the guilt he's feeling about possibly beating an opponent with a sick brother? By visiting said brother in the hospital and showing him the Build Burning. By the end of the episode we see that it's inspired the young man to go beyond straight builds and work on his own custom Gunpla, just like "Mr. Sekai"...and he still wants Shimon to use them.
Imagine you're a sick little kid stuck in the hospital. Suddenly, this cool guy comes to visit you, shows you his totally awesome looking gunpla, and when your brother shows up, says he came here to DECLARE WAR. Your brother, and by extension your gunpla, are doing so well that you now have the respect of high-tier opponents. Also, Sekai established that he didn't make the gunpla in question, making it known that there's other teams out there using the "one builds, one fights" method, just like you and your brother.
Episode 8
Sekai defending Gyanko when she is called out by Yomi for "being on a date" and not thinking about the upcoming match.
Episode 9
Fumina goes and tries to comfort Gyanko after her defeat to Team G-Master.
During the stinger of Episode 9, Fumina personally visits Sekai, who is still sleeping off his injuries, to tell him thanks for bringing them to the national championships.
Mirai especially asked for a day off to cheer for her brother.
In the final move Sekai using the Lightning Gundam's jetpack, the core fighter of the Winning Gundam and Sekai entering Burning's Super Mode, states Team Try Fighters are all connected.
Episode 10
Sure, one may consider Yuuma's attempts to woo Mirai as comic relief, but going through such lengths just to teach her how to build her own Gunpla is heartwarming in itself.
The icing on the cake: Yuuma installing the petit Beargguy on it, making Mirai squee. Ricardo Fellini would have been proud.
Fridge Brilliance as well: When his elder sister China started to become close to Sei, what was the Gunpla that kickstarted it for her? A Beargguy. And now Yuuma's aparently continuing that with Mirai, as well...
It is nice to know that Mihoshi was able to fulfill her dreams between the time skip.
Sekai having complete faith in his big sister was very heartwarming, showing that they both love and respect the other.
Mirai reciting the creed of the Jigen Haoh School to Yuuma, bouncing back immediately from her loss. Sekai might be the stronger fighter, but Mirai clearly has mastered the philosophy of their teacher.
Episode 11
Sekai singing the 1st OP for Mobile Suit Gundam ZZ with Fumina is cute in and of itself, but it also means that he's been watching the series' with her.
Episode 12
Meijin Kawaguchi takes the time to act as a Warrior Therapist for an understandably depressed Yuuma, likely also looking out for him in China and Sei's absence.
So did Lady Kawaguchi to Fumina. Granted she was a bit rougher than the Meijin, but she got her point across...
Episode 13
Yuuma admits he doesn't like Sekai very much, but he acknowledges him as the team's ace.
Episode 14
Gyanko and Shimon showed up to help the Try Fighters in upgrading their Gunpla (Well, in Gyanko's case it's just Sekai). True Companions indeed.
Also, Lucas has in fact taken Aila's words to heart.
Episode 16
Sakai's bonds with the rest of his team. He states on TV that above all else he wants to play Gunpla Battle with his team. Considering how much effort he puts into his rivalry with Yuuma, that's a huge deal.
While that is going on, Yuuma gets a phone call and steps outside to answer it... and it's China, telling him that she and Sei are cheering him on.
Kodera's withdrawal from Team Build Busters in order to allow Minato to fight in the nationals has also given the latter a sense of duty and responsibility in fighting, beyond his rivalry with Yuuma. The fact that he seems to have begun making friends as well makes a very positive development from the smug Jerkass we saw so far.
Episode 18
Yuuma is initially worried about repairing the Build Burning, because he doesn't believe he's at Sei's level - and then he gets a call from China, who's at the airport to give him something: spare parts for the Build Burning. That's not all, she has a message from Sei to Yuuma:
China: The Build Burning was made for his precious best friend, and customised so that person could handle it better. He sealed it away with his memories. But he was very happy to see it moving... Yuu-kun, tell Kamiki Sekai: "The Build Burning is your gunpla now. And someday, let's play Gunpla Battle together!"
In addition to the above, China is holding an exhibition of her art, but took the time to fly to Japan to deliver the parts and the message, for both Sei and Yuuma's sake. And not only that, but the plane she used? A Yajima Trading corporate jet, meaning that her rival Caroline helped her. It's subtle, but it shows how Caroline has softened towards her compared to 7 years prior.
After the battle ends, Wilfred approaches Team SD-R. He tells Toshiya that he doesn't know why the Gunpla Academy didn't accept him, because he is an excellent builder, and he was hoping to have a match with him someday. He then asks him to not hate Gunpla Battle, to which Toshiya replies that he doesn't — he wouldn't feel disappointed if he did.
Episode 20
A light example: After Junya throws Sekai to the ground, Fumina and Yuuma get behind Junya, implying they are going to attack, but Fumina makes it clear that they are only there to prevent Junya from running away from the fight. They both then tell Sekai to get up, believing he hasn't lost yet.
Later, when Sekai is on the verge of giving up, the Try Burning responds in it's own way, essentially telling him not to.
When Junya is walking towards Sekai in the aftermath of their fight, Yuuma assumes Junya is there to attack Sekai and stands in his way, even though Junya turned out to be there to patch things up with Sekai. Given how much Yuuma claims Sekai irritates him and how Yuuma has said he's not very good at actual fighting, it's a really nice moment.
Akira deciding to give Junya a ride to the station after the battle, and Junya agreeing shows that their past grudges with each other have been buried. Akira even suggests they make a detour to play a quick match, when before he bluntly said he'd never play with Junya again.
Episode 22
Lucas and Wilfrid acknowledging each other as Worthy Opponents, then followed by Lucas thanking Aiba and Tomita for their support, and then telling Granada's coach that they carried out his plan splendidly, and the only person to blame for Granada's loss is him.
Minato's teammates encouraging him to fight to the finish. Accenuated by the "Allied Force" playing in the background, a subtle reminder of the bonds they formed as teammates.
Episode 23
Shimon meets up with Kaoruko and her team for the final match, accompanied by his brother Mamoru, who has recovered enough to leave the hospital.
Episode 24
After all of this time, all the hard work and sacrifice not only did Team Try Fighters finally make it to the top, Wilfred obviously had a massive blast with the fight.
Not to mention Yuuma finally delivering Sei's message to Sekai.
I'll be waiting for you at the World Championships!
Episode 25
Pretty much all of it. From Mamoru losing his division, but gaining a rival, Gyanko's little brother at that, to absolutely everyone jumping into the duel between Yuuma and Minato and having a blast. Hell, Ramba Ral even finally got that Dom, even if it's a different Ral in a different universe.
Special note goes to the moment Fumina enters the exhibit hall, where a girl runs up to her and expresses her admiration of Fumina, wanting to be a battler like her, not unlike how she herself was to Lady Kawaguchi, bringing Fumina full circle.
The ending sequence shows Yuuma being mobbed by new students interested in the Gunpla Battle Club, to Fumina's delight.
Um, that may not be the club they were interested in.
Also, Mamoru has gotten better, and he and Tateo are hanging out in the park, happily talking about gunpla, while Gyanko and Shimon look over them fondly.
Best of all: Master Chinan is still alive and is still teaching Shingyo-Ryu disciples.
The Island Wars OVA
Fumina's mom being a total Wingman, and recruiting Sekai to accompany her daughter as a test pilot even as she was bummed from Yuu passing it off.
The credits, The Stinger and the final scene. The mysterious girl is revealed to be Reiji and Aila's daughter, and she happily greets her mother upon returning to Arian.
Also the final image of the girl's Gunpla, along with Reiji's Beginning Gundam and Aila's Command Gundam.
Mind you, the last time we saw the Command Gundam, it was on Earth. This means Aila (and/or possibly Reiji) had to have gone back to get it from China.
Possibly foreshadowed. Back during the finals, when China calls for the unseen Sei that they're starting, there's a quick cut to four teacups setup on their coffee table. While it could easily have been for Sei's visiting parents (China and Yuuma's were shown to still be in Japan), that may have been the first hint that Reiji and Aila had indeed been able to return, and that's when Aila got the Command Gundam from China. And seeing as Reiji does show back up in Gundam Build Fighters: GM's Counterattack...
A subtle one compared to the rest of the OVA ; In the aftermath of the three great superweapons firing, when all hope seems lost, and the Build Fighters are at their weakest Sekai stands up above the sky of Sydney and prevents the first, most fundamental, most devastating, senseless loss of life in Gundam history. It's virtual and entirely symbolic, but it's one of the most powerful moments in Gundam history that was changed by the willpower of the protagonists and they deserve it.
