When Raimei and Raikou fight for the first time, Gau is listening to Strauss’s “Unter Donner und Blitz", which is German for “Under Thunder and Lightning”. Incidentally, Raimei's name means "thunder", while Raikou's name means "lightning". And what happens "under" thunder and lightning is a rain shower...in other words, Gau.
The song is also the reason the word "polka" is in every chapter name in the arc.
When Raikou swears that he's going to kill Yukimi, the next panel shows a bolt of lightning. The panel after that shows a pouring rain shower.
If you look closely, Kouichi is actually putting his hands up in the "rock on!" sign when he's being held hostage by Kannuki at Alya Academy. He seems weirdly carefree for someone who's got a gun to his head, but considering that Kouichi is immortal, it makes sense.
Speaking of his immortality, the claim that he is 14 years old on his character sheet is technically correct. His persona as "Aizawa Kouichi" is indeed 14 years old.
His blood type on the character sheet is listed as "unknown" because he's an owl. This also accounts for his taste for rabbit meat.
The thing that Miharu trips on while running away from Alya students in chapter 32 is actually Shijima under her cloak in cat form.