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.
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.