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.
YMMV: Brave Frontier
Anti-Climax Boss: Most bosses can be easily beaten when you know their attack patterns.
Crack Pairing: Douglas x Serin: Despite having no interaction in their lores, were paired off sometimes due to their virtue of being Spark King and Queen.
Foe Yay: Luther and Douglas, with the former was the one who killed the latter.
Mega and Stya probably defines this trope as the latter attacked Uda in rage after he wounded the former.
"Brave Burst spam" and "Critical Hit spam" team archetypes. The former is designed as such so that all of your units can use their Brave Burst every turn. The latter is designed to make full use of Mad God Zebra's Leader Skill, which increases damage from critical hits to absurd levels, by increasing the critical rates of your units as much as possible.
Felneus can also do this when combined with units with high hit count like Douglas and Serin due to its Leader Skill (which boosts the effect of Brave Crystals) and Super Brave Burst (which increases the drop rate of said Brave Crystals).
Goddamned Bats: Unicorn and Pegasus have an incredibly long attack animation. The paralysis effect they carry with their normal attacks can easily bring them to Demonic Spider tier.
Memetic Loser: The Thunderbird line, despite having a great improvement of the 5-stared Zazabis stat-wise, is still labeled as the worst units and are the last one people want to receive from Rare Summon. Then again, Zazabis weren't grouped together with its lower counterparts in their receiving end of mockery.
Ship Tease: There's a few of this in the unit's flavor text.
Zeln/Nemia: Certain on Zeln's side, considering that his 4 stars and 5 stars evolution stated that he focused on training himself to follow her.
Vargas/Lava: Small, but there. Also crossover with Foe Yay, considering that she defeated him before opening up to others, and in her 5 stars evolution it was stated that she "wanted a rematch with Vargas even more than a battle with the gods". Also, Vargas's 6 stars evolution outright stated that the wings on his back were given to him by Lava.
Loch/Edea, at least one-sided, considering he chose to come out of his self-imposed exile to join her.
That One Attack: Sir Sancus Xenon Lv10's Area of Effect attack Soul Desecretion, especially when he casts it thrice in a turn. That attack can and will most likely kill your entire squad if you're not guarding.
Any time you're facing Phoenix. It constantly uses its Brave Burst, which lets it regenerates for a few turns before using it again. Unless you have poison or paralysis, it'll either be a long or impossible fight, depending on whether you can damage it more than it recovers.
Miracle Totem. Not only it can cast Brave Bursts of different elements, it also has a new move, Chaos Trilogy, that can, together with using other attacks at once even knock out 5 star units!
Thunderbird and Great Falcon Ziz due to their both their bad stats and Brave Bursts attacking random enemies instead of all. They are also flat-out scrappies as they can be randomly summoned from the Rare Summon, especial when players expect better units. Zazabis, on the other hand, gets the underrated treatment. Its stats are better than expected, so at least it's usable on boss fights that are only one enemy.
And among the ones normally obtained in-game, healing units are basically the inversion. Titania and Legotes are special mentions for being easier-to-obtain (though inferior) versions of God Tree Eltri and Phoenix, respectively.
Viewer Gender Confusion: Official flavor texts explicitly refer to Atro as male, but there are many who would insist that "he" is actually a "her." The confusion was so bad, even the translator dubbed his 5-star name as "Goddess Atro" before the term was corrected in a recent version, and even then, it took further updates to correct all his gendered terms. His somewhat androgynous appearance certainly does not help.