If you think about it, Karen is the greatest antagonist so far: it doesn't matter if it was intentional or not, but she has placed her brother in terrible situations multiple times, he had to get serious after the results of her actions (think of heroes powering up after the bad guy harms the hero's beloved, except here Karen is both the bad guy and the beloved) and now a flashback tells us she made an agreement with some powerful being to do something that is clearly against the main character's wishes. She is the most diabolical character so far.
On a different note, I don't know if it has been pointed out in previous chapters, but some names have some interesting characteristics and they might or might not be important, so here is a list of those I know of: "saijou", even though it's written differently, means "best" and indeed the Saijou household is supposed to be (one of) the most powerful families; the academy is called "shishiouji" which means "lion prince"; "shiroouji" is literally "white prince" and in fact the guy has white hairs; the last one looks more like a trivia than anything, but "mizuha" and "shizuka" have in their names the kanji respectively for water ("mizu") and fire ("ka").
I haven't researched the names of the other characters at all.
Lastly, only a .5 is left before catching up with the raws.