Ren, and later Aichi's repeated statement of "Despair to the powerless, and all glory to the one with the power." throughout Season 1 seems a bit odd, being essentially just a standard Might Makes Right statement. However, it makes perfect sense considerhing who says it. Both Aichi and Ren fell into despair from their own powerlessness (Aichi moreso than Ren) and only reached "glory" through Psyqualia. It's not just an insult for their opponents, it's a reflection on how they got to where they are!
One criticism of the Gold Paladins when they were first released was that they're a Suspiciously Similar Substitute of the Royal Paladins. But when you realise that at least two of their leaders were canonically part of the Royal Paladins (Garmore and Ezel), it makes more sense. Of course they worked similarly to the Royal Paladins, they were trained and led by warriors who are Royal Paladins!
Ren and Kai act very differently from their Season 1 selves during the second season which makes perfect sense considering that they have forgotten Psyqualia and the events surrounding it, which was a core point of their relationship and personality.
Kai's turn to evil in Season 3 makes more sense when you look at his record in Season 1. He never actually beat Aichi or Ren in Season 1 except in flashbacks. He would have likely lost his fight against Psyqualia Aichi had the fight continued, given the amount of shield Aichi had left, and his attempt to defeat Ren ended with his 'final evolution' being crushed by Phantom Blaster Overlord. In Season 3, he has that as his established record against the two. Why doesn't he try to get stronger by practising more and building up his own strength? Because he tried to do that before, and it didn't work. So when he's fighting Takuto and facing Mind Rape, his faith is already compromised by the fact that his reliance on his own strength hadn't won him the fights that truly mattered. Between the Mind Rape, knowledge that his faith has gotten him nowhere, and the power he's being tempted with, it's small wonder that he gives in.
Towards the end of the 5th Opening, Eradicator, Vowing Saber Dragon "Reverse" and Silver Thorn Dragon Queen, Luquier "Reverse" appear with Star-vader, Infinite Zero Dragon. Why these two in particular? They're the first two Reverse units to be shown in the anime.
And towards the end of the 6th Opening, Eradicator, Vowing Sword Dragon and Silver Thorn Dragon Tamer, Luquier are seen getting shot down and falling back towards Earth.
Suiko's morally questionable actions throughout season 1 make a lot of sense once you consider season 3's reveal regarding her employer Takuto and his method of Recruiting her and the rest of the Ultra Rare and the lengths he went to involving their memories to ensure their assistance. This in turn also explains why Kourin's apparent "spanner in the works" never really goes anywhere as she is in the same situation as Suiko, therefore if she wants her memories returned she must remain a loyal ally until the mission is concluded.
Why Kai is so certain that Ren remember Aichi in Legion Mate. Ren is still his nice self, not his psyqualia-mad self. Since the current Ren is a result of Aichi beating him, and Ren hasn't reverted, he must remember.
The card Star-Vader Blaster Joker. At a glance it looks like just a combination of Blaster Blade and Link Joker, but considering the Tarot meaning of the Joker (the Fool) it takes on a new meaning as the Fool/the Joker represents the "protagonist" of the Arcana.
For the card game itself
While Grade 4 units are commonplace now a days, a few units are able to become Grade 4 units as Normal Units and not G-Units.
Epitome of Knowledge, Silvest has lived for several millennia and is known to train his mind and body thus achieving a level akin to a Grade 4. Appearing once every century or so, he comes to encourage young students but is very illusive thus why his card has an expiration date.
Nubatama’s Afterimage: The Nubatama are ninjas. After creating a division (binded opponent’s card return to hand), the Ninja have enough time to prepare an illusion (afterimage). However they need to act fast to prevent their illusion from being discovered as fake. Since in anime logic, lighter load = more speed (hence requirement of having less than 6 cards in hand).
Neo Nectar’s Bloom: Flowering (bud become bigger due to petals opening up, represented by two unit with the same name), turning a bud into flower (gaining new effect).
Spike Brothers’ Charge: Spike Brothers are based on American Football, in charging state basically means they are running towards their opponent (to tackle them).
Link Joker's Lock: They're alien invaders, thus they keep their target in stasis to analyze them and keep them away while preserving their being. After being integrated as a clan of Cray, they also use it on themselves to keep themselves in stasis, to preserve energy and devise a strategy, thus gaining abilities upon unlocking.
Deletor’s Delete: The vanguard’s attack and ability become blank, basically, you are deleting the text in the ability box as well as their power level in the card.
Deletor's Vanish Delete: Basically deleting unit from existence (presumably, permanently).
Tachikaze’s Engorge: Since Tachikaze are dinosaur... for fridge horror: the reason why rearguards retired is because the said-unit with the said-ability, ate them. Being Engorge state basically means they are satisfied (from eating/retiring the rearguard), hence have the energy/motivated to do other stuff (activating another ability based on how many unit retired/eaten).
Bermuda Triangle’s Harmony: Bermuda Triangle are idols, harmony is basically an idol group/duet (having two units with harmony ability on same column/stage).
Granblue’s Hollowed: Japanese Kanji used for the keyword is Apparition and since Granblue are basically undead, retiring at the end of turn after being in hollowed state meaning they are returning back to grave (after returning from it).
Pale Moon's Magia: Stage Magician should come into mind, Magician declared a hat-pulling trick but you don’t know what would the magician put out of his empty hat (able to call any unit from soul to rearguard circle) but a magic trick is ultimately still an illusion anyway (return to soul at end turn).
Oracle Think Tank’s Oracle: the idea of having more cards in hands(5 or more cards in hand to activate ability) means that they are prepared for their opponent after foreseeing the future.
Nova Grappler’s Rush: The japanese Kanji of this keyword is: Fighting Spirit, basically, Rush is a heroic second wind (effect trigger after the Unit with the ability, Stand due to other effect).
Narukami’s Thunderstrike: They restrict their opponent’s movement (binding them) so as to strike them with lightning.
Gear Chronicle’s Time Leap: They can travel through time. Binding a unit means using the unit to call for reinforcements for a Unit of higher grade from the future (hence from Deck to field).
Why only one Grade higher (chain of command, getting help from their immediate Superior. Having Commanders means Gear Chronicle do have some form of Hierarchy)
Why the Higher Grade Unit return to the bottom of the deck at the end turn?
Because they just went to the past (stuck down there), it will take a while for them to return to the present by taking the long way back. Of course, you can always use time-leap again to get their lazy ass back quickly...
Aquaforce Wave: Aquaforce is a military unit, their multiple attacks probably represent them attacking in waves (new ability after multiple attack because the previous waves are merely to buy time to get another group to come in with better equipment more suited to deal with their enemies.).
Royal Paladin's Brave: Considering that Brave activates when the player has three or less cards in hand, they become more powerful to protect what little reinforcements they have left.
Kagero's Blaze: Fire becomes stronger the more area they cover, translated in the game, the more you have on the field, the stronger they become.
Gold Paladin's Unite: As sort of the direct opposite of Royal Paladin's Brave, having more reinforcements means they can go all out and can obviously work together to become stronger.
Angel Feather's Rescue: Medics would rush in to get one of their injured out of danger, but in doing so this would put themselves in danger as well, thus why their Vanguard takes damage.
Dark Irregular's Darkness: Vampires would sustain themselves by constantly drinking the blood of the living, thus why darkness deactivates at the end of turn since it would only sustain themselves for a small time.
Dimension Police's Burst: Superheros constantly push themselves to the limit and upon getting stronger, they may also gain new superpowers. If we apply this to the Dimension Police's Mecha, they unlock more features the more they are powered up.