According to the in-show profile, Greco Logan is twenty-seven years old and has been a Gunpla modeler for twenty years. If you take into account this series' probable setting (somewhere in the late 2010s or early 2020s), Greco could possibly be a Winger, an American Gundam fan who literally started out with Toonami's airing of Mobile Suit Gundam Wingback in 2000. And considering his build, skills as both a modeler and a fighter and overall badassery, one could almost say Greco is vindication for such fans, who were heavily discriminated against during the period.
Bonus points go to him using a customized Tallgeese while his Friendly Rival uses a Wing Gundam.
The age of most Fighters aligns with what they use and what was popular when they were young. The older veterans like Universal Century suits, Fellini and Greco use Wing suits (being 90's kids), and the younger generation likes to use AU's from the late 90's and the 00's—namely SEED and 00.
Nils Nielsen's being able to control Plavisky Particles so well makes sense due to his PhD in Particle and Energy Physics and that he has been studying Plavisky Particles for some time, which is his motivation for taking part in the Gunpla Tournaments in the first place. Additionally, his obsession with samurai and martial arts might come from that his mother is a martial artist.
The match we see in America is so much more melodramatic and that one announcer is so easily excited compared to the other one. Why? The announcers are modeled after American sports announcers: One does play-by-play, explaining to the audience what's going on, while the other is the color commentator, the one who typically with offer observations, crack jokes, and generally add flavor to the commentary (think John Madden). And the Fighter's intros? Straight out of Professional Wrestling.
Why did Sei suddenly become so much better in Episode 23? In Episode 25, Reiji said Sei was never bad at controlling, that he was just worried about damaging the Gunpla he put so much work into. Since neither the Beginning Gundam nor the Build Gundam MK-II were in the tournament, there would be no repercussion to damaging them. Also, the MK-II was no where near as customized when compared to the Star Build Strike, which he poured so much time into, and he didn't even build the Beginning Gundam, so he had no personal attachment to it.
That damage to Wing Fenice's head that Fellini never repaired? In an easily-missed moment, the article about Tallgeese Valkyrie that Nils is reading has a bit about how the match between Fenice and Valkyrie in the 6th World Tournament came to a standstill, so the battle was switched to "V-Attack", basically a Sudden Death mode where the first one to inflict damage on the other was declared winner. The damaged eye and V-Fin were likely from the blow that made Fellini lose, and he kept it as a reminder.
Sazaki's apparently random appearance at the all-girls Gundam tournament makes sense in the following season, where his younger sister is easily more badass than him in combat. Clearly he got her started early.
A lot of fans reacted oddly to Yuuki/Meijin's third Gunpla being an Exia variant. However, it was foreshadowed in his previous Gunpla: his Zaku Amazing is a parallel to the Avalanche Exia pack, and the Kampfer Amazing's weapon binders look like the GNHW/M suite from the Arios Gundam.
The gathered Build Fighters managed to take down the Mega Size 1/48 Zaku-II - it's stated a few times that the heavily customised models have a huge advantage over unmodded straight builds. And close inspection of the Zaku's instructions reveal that its complexity has been deliberately lowered to that of a HG - same as the Build Fighters.
Why did Aila not dodge the Build Knuckle, when she could literally sense the particles? Not only had she thrown away the helmet connecting her to the Embody System by that time, but the sheer amount of Plavsky Particles given off by the move caused her to suffer Sensory Overload, not unlike having a flashbang go off in your face.
In Battlogue episode 1, why didn't the Reversible Gundam have its Twin-Drives or use Trans-Am against the Ballistic Zaku? Because much like the Ballistic Zaku it is based on the "Origin" variant of the Reborns Gundam from Season 2, which lacked both of those features before Anew Returner delivered the Twin Drive System's data.
What if the "Gundam Valhalla" concept didn't stop at just the characters we know? What if Sei, China, Rinko, Takashi, Fellini, Yuuma, Fumina, Sekai, Shimon, Luang, the Sakakis, and everyone else were among the billions of nameless civilians who have died in the crossfire of all of Gundam's wars?
If Chairman Mashita is supposed to look like the older Tem Ray as we know him, then Takeshi Iori, Sei's father, resembles a younger Tem. Just like Tem, Takeshi is away from home a lot and has obsessive behavior related to Gundams, which would've been a strain on relationships with anyone who didn't share the same enthusiasm. Fortunately, Takeshi is also a doting father to his son, but this just shows the sort of relationship Amuro could never really have with his father.