The reason Asahi sometimes calls his three friends "Oka", "Shima" and "Sakura" is because their surnames all end in the same kanji so leaving it off would distinguish them further. That and the Shout-Out to Masuda's previous series explained on the character page.
Every character has hair colors that corresponds to their names, except for Shiragami Youko, who has the kanji for "white" but has light green hair instead. But then you realize the color refers to her father Shiragami Genjirou, who indeed has white hair. Adding to that, her mother has dark green hair. White + Dark green = Light green.
Mikan joining the others in chapter 92 mocks the Always in Class One trope, but may actually be justified. Considering Akane directly manages the non-humans in the school, it would make sense for her to regroup them all in Akari's class. And since Mikan's glasses became a God of Fortune somewhere during the previous year, she was put in the "supernatural class" for the new year.
Rin calling Mikan "Granny" is a Red Herring designed to make us think that Asahi marries Mikan in the future - while it's not stated for certain, it's pretty obvious that Mikan is actually Rin's grandmother on her father's side.
Drinking blood being akin to an intimate, sexual act for vampires in this series probably got inspired from the classical Dracula, where his attacks on women are metaphorical rapes.
Okada's actions in chapter 2 make much more sense when you know that he can hear people's thoughts. He's aware of Youko's secret and that Asahi is keeping it, and he's also aware of Mikan's mindset, which is why he's able to give Asahi the perfect advice to derail her plot.
Jossed. Oka reveals in chapter 190 that he reasonably guessed something was up, but never learned Youko was a vampire until the big Internal Reveal.
The eventual revelation that Akane's ultimate goal is peaceful coexistence of humans and non-humans makes a lot of sense in light of the fact that it's extremely likely that she herself (and by extension, a number of her descendants) had a romantic relationship with a human at some point her life, and had to personally deal with all the inherent difficulties of such a relationship. It's an issue that would directly affect not only her friends (Genjirou, Touko, Shirayuki), but also her children, grandchildren, etc. Similarly, her skill at manipulating people's emotions isn't just a side effect of her telepathy; she's had hundreds of years of experience dealing with these sorts of situations.
