These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Anticlimax Boss: A bit. Even though the battle between Reiji and Sei, and Mao was hyped up for a good chunk of the show. Their final battle was a bit of a Curbstomp Battle. Justified a little since Sei had made numerous additions to the Star Build Strike since their imagination battle, when it still was only the Build Strike Full Package, while Mao didn't really have any additional improvements to his Gunpla aside from the Maoh Sword.
"Allied Force", which perfectly fits its use in Episode 15, chronicling Fellini & Fenice's history together, and their Heroic Second Wind. Also used during the climax of Reiji vs Nils battle.
When "1/48" plays, you know shit is about to get real. This perfectly fits the intimidating nature of the 1/48 Mega Size Zaku II. It's also a striking track during the first half of Reiji vs. Aila in Episode 21.
"Aila" is nothing short of a beautiful, emotional theme for said character. The heart-wrenching tone of the song works incredibly well when Reiji learns the truth about Aila.
Chairman Mashita is pretty divisive as well. Some really don't like his Manchild tendencies and his increasingly petty actions against what are basically a couple kids. While some love his exaggerated expressions, especially when they involve encountering Reiji. May be an example of Love to Hate.
Broken Aesop: Semi-Averted. The aesop about the Renato Brother's tactics might seem to be about how they fight dirty, even when other opponents use the same exact tactics. However, it's more about their attitude towards Gunpla Battle: They're douches, and in real life they'd probably treat casual play like a tourney, sneer at you for not playing Smash Bros the right way, and use phrases like "metagame" and "mindgames" ad nauseum.
Sei's relationship with Reiji and Sazaki. Sei didn't want to team up with Sazaki purely because Sazaki allegedly treats his Gunpla like garbage and always gets them damaged, even though he is seen constantly improving his Gyan through the series. Reiji also fights in a way that can damage the Gunpla and Sei repeatedly gives him his blessing, reassuring Reiji that damaging the Build Strike and the other suits is perfectly fine since he'll just repair it. So then... what was so wrong with working with Sazaki then?
Probably the fact that Sazaki is a huge prat. He wants Sei to partner up with him, using his Gunpla to win, but he expects Sei to do so after taunting him about his lack of combat skill, utterly humiliating him in Gunpla Battles, and acting condescending to Sei when the boy's trying to make a sale to customers? Sei can make any kind of justification he wants for not wanting to partner up with Sazaki, but it's fairly obvious that in the end that he doesn't want to because Sazaki's kind of a jerk.
Aila's popularity exploded after Episode 10 showed that she was Not So Stoic and had Hidden Depths. It also helps that she's the equivalent of the series' female Newtype lead, who have traditionally been very popular among fans.
And now Chairman Mashita's secretary joins the club due to her attractiveness and ability to deal with Mashita's childish behavior.
The little girl shown with Sazaki in the last episode (presumably his sister) became disproportionately popular on /a/, with many posters hoping she gets more screen time if a second season ever airs. Note that this character appeared for only several seconds and received no name or characterization.
Also, she had a tiny Gyan hat. How is that not utterly adorable?
That little girl is, in fact, Susumu's sister Kaoruko. She comes back in Gundam Build Fighters Try as one of the main rivals.
For a minor character whose significance only lasted one episode, Greco Logan also has quite a fanbase for himself. Enough at least to warrant cameo appearances in the last few episodes, in which he featured rather prominently.
For the inevitable Gundam Yaoi Fangirl demographic, there's Sei/Reiji, helped along by their Ho Yay in-series.
Rinko Iori/Mr. Ral emerged from their status as the only two adults with a significant role in the early parts of the show, the latter's (unrequited) canon crush on the former, and the fact that they spend more screentime together than Rinko does with her actual husband.
Allan/Meijin seems to be the most popular ship involving Meijin Kawaguchi aka Tatsuya. It helps that unlike the other main characters, Meijin doesn't get a female love interest and Allan in some ways serves as The Not-Love Interest for him.
Harsher in Hindsight: Aila's love for food is revealed to be at least partially based on the fact that she was impoverished as a child.
Hilarious in Hindsight: Sei fixes the Build Strike's shattered arm in time to battle Nils, and it holds up stronger than before - because Sei used Kragle!
Ho Yay: Between Sei and Reiji, "partners" in Gunpla battling.
The first opening theme doesn't help much.
Lyrics: "From the minute I felt you in my heart, I was sure my dreams would come true."
Sei exhibits Tsundere tendencies when dealing with or talking about Reiji.
Honestly, does Allan really have to follow Meijin everywhere he goes?
Interestingly enough, in the last episode there's a montage of all couples (Nils/Caroline, Fellini/Kirara, Takeshi/Rinko, etc), living happily one year later. In-between all those, there's a shot of Yuuki and Allan working with kids.
Marty Stu: Nils Nielsen, in the same vein as a newbie's first RPG character. Holds several PhDs from "America's top university", black belts in karate and judo (courtesy of early training from his martial arts master mom), has only three months of Gunpla building experience yet builds a top of the line model that completely decimates a Gunpla built by a 20 year old vet, and he performed the latter boasting of his superiority and making light of his opponent's attempts at fighting (even though it was clear said opponent, Greco Logan, was the more skilled fighter). Oh, and did we mention he's only thirteen years old, such that his nickname is the "Early Genius"? Combine that with the Anvilicious samurai theme for both his Sengoku Astray (one imagines even Lowe Guele would have found the design over the top) and himself (he even fights in a gi and hakama), and one has a disturbing feeling that Nils is a huge parody of or a Take That to the Western fandom at large.
Moral Event Horizon: Josef Kankaansyrjä wastes little time crossing this in Episode 20. While it's one thing to take your grandchild out to see Gunpla fights, it's another to have Aila's handler raise the power of the Embody System to max output. For the sake of entertaining his spoiled grandson, Josef mind rapes Aila to the point of hitting a mental meltdown and utterly destroying the Gundam Fenice after Ricardo concedes the fight. Worse still, he nonchalantly states that he has replacements should Aila fail from the device...
Nine Barthes crossed this before the tournament began by designing and producing the above-mentioned Embody System. This moment proves beyond all doubt that Nine views Aila not as a person so much as a tool for his company to win tournaments.
And then he's hired by PPSE to develop a version of the Embody System for the Meijin.
Not to mention that Build Strike and Star Build Strike's beam rifles make the classic beam rifle firing sound from the original series. Many other audio cues from other shows appear, like Exia's boot-up noise.
"Beginning Plavsky Particle dispersal."
"Everything's gonna be alright."The line of the first opening theme song that plays when Sei and Reiji free Yuuki from the Embody System and win at the same time in Episode 24.
Narm: The Meijin's over the top wardrobe change at the end of Episode 23 had many fans finding it difficult to take him seriously.
Narm Charm: Yuuki-as-Meijin has the habit of making very Narmy remarks in all seriousness.
Rescued from the Scrappy Heap: This series seems to have a good track record of making every recurring character that was originally detestable likable. Examples include Kirara and Nils.
In Nils' case, he has steadily come out of his shell since his debut. Getting involved with some great gags and antics with the colorful cast, as well as developing to show his valor and fandom of Gundam and Gunpla Battling, even so much as losing his composure after his beloved Astray took heavy damage. Not to mention his consistently awesome gunpla kills.
For all his character flaws, Fellini's fight with Reiji and Sei in Episode 15 showed that he is one helluva Gundam pilot and Determinator (even more so than his other rival Greco Logan). Especially given this sequence◊. Plus, it also helps that Fellini has transitioned more into The Mentor as the series progressed, such that he's one of the few people who has a degree of influence on Reiji. This came to a head in Episode 21 when Fellini baited Reiji (who was still anguished over Aila's "betrayal") into a fist fight so that Reiji could vent his rage.
Then, after calling Fellini out of his self-detonation attempt in Episode 20, Kirara is again getting flak from some fans (thus subverting this trope), mostly for having denied a supposed send-off for Fenice (and probably a homage to Wing, too. But thanks to her, Fenice comes back a few episodes later with a cool redesign, just in time to take part in the final battle... so it probably evens out.
Strangled by the Red String: Averted with Fellini/Kirara and Aila/Reiji. Unlike many installments in the Gundam franchise, Build Fighters takes its time to develop the relationships and has both couples going through highs and lows. For many fans, this makes their respective pay-offs (Episodes 20 and 21 respectively) all the more heartwarming.
Straw VulcanHas A Point: Sei treats Nils' offer to forfeit in exchange for help in learning more about Plavsky Particles as an absolutely underhanded, disgusting action... even though Nils himself raises the accurate point that it could revolutionize any number of sciences and improve living conditions on Earth. And sure enough, it's purely because of him later on that the Gunpla Battles can continue, because Reiji let him study his pendant.
Tainted by the Preview: Exia Dark Matter and Miss Sazabi were not well received when previews of the high grade model kits were shown. Subverted later on when the fandom warmed up to both mobile suits when the animation version of both suits were shown to have much more detail than their model kits.
Vindicated by History: Look at the comments on the episodes on the GundamInfo channel, particularly episodes heavily featuring suits from Wing and other AUs. Wingers, SEED fans, and other fans of the AUs are coming out of the woodwork, thrilled that Sunrise-Bandai are acknowledging the AUs. While UC STRONK annihilates the unnamed cannon-fodder AU suits, many of the named characters use AU suits which evens it out. Plus the protagonist uses a modified suit from SEED, and several of his significant rivals use suits from SEED Astray, Wing, X, and 00. Plus, Sei and Reji's final opponent in the tournament? Exia.
Some of the AUs and non-animated UC side stories that have shown up: SEED, 00, G , Wing, X, Sentinel, Blue Destiny, Turn A, G Saviour, Crossbone... with special guest appearances from Detonator Orgun and Aura Battler Dunbine.
On the specific character: Nils Nielsen, after his overtly embellished debut in Episode 8 causing controversy, Episode 9 proves that even the resident Marty Stu is not immune to zany situations they're not accustomed for, made him fairly more likable. And later into the series his weaboo tendencies are toned down in favor of him being the eternally-exasperated straight man.
Wing Gundam and its model, specifically the original 1/144 version. Seeing how Fellini basically got a base 1/144 Wing Gundam which is not even completely HG, and keep using Fenice and repairing it through out at least several decades even though, better models, have came up brought many Manly Tears, and also caused many fans to dig out their old/forgotten first Gunpla.
The series also did a good job in restoring the franchise's reputation following what happened with Gundam AGE.