I have to disagree a bit here. Himari has been neglected since nearly the beginning. She really needed an episode of development. I like the way they tied it back in. While I'm still very much interested in Sho's fate and Ringo's ultimate reaction to her new partner in crime sacrificing himself for her, you'd have to wonder what Himari does at home all day other than work out and knit. She can't let her brothers sacrifice everything for her sake all the time. She's a very important character, despite recent events given the impression that the only main characters are the younger of two brothers and a psychotically delusional stalker.
I also wouldn't say it failed to reveal anything. I also can no longer say the reason she was miraculously saved from her illness be the simply random chance of buying her that ridiculous penguin hat.That aside, I didn't really feel like this ep revealed anything much of importance. It seems to be what Himari experienced prior to resurrecting wearing the alien hat in the hospital. They didn't even bother to tell why she left the school before Trible-H was formed. Did the parents die at that point?
Now I can't help but wonder how much of The Princess of the Crystal is really Himari. She was best friends with a pair that eventually became very popular idols. I knew from the beginning of their flashback that the two would turn out to be the same idols used in chibi form for the train mottos. Perhaps the Princess of the Crystal isn't some external supernatural being, but is merely another variety of Himari. Her clothing is idol-like, her intro is over the top like some giant concert, and she does choreographed moves as she descends to harass and insult Kanba and Shouma.
When the 'librarian' put on her "veil" she remained very Himari-like, not the proud and rude personality we've gotten used to seeing. At the same time, she wasn't as passive as Himari usually is. To me, it is starting to feel like the Princess of the Crystal is the embodiment of who Himari wants to be. Loaded to the brim with the confidence she didn't have in the past, showy, and an idol. The incident with the koi may also explain some of the "Survival Strategy!" dialogue. It's certainly in no way as simple as, "they're both the same person." Something supernatural and crazy is definitely going on, but it has become more difficult to say that the two are entirely separate individuals now that we know Himari's past and her younger dreams and ambitions.
They never said who threw the eraser, but the two girls looked sad, not angry. We know that Himari's health declined very rapidly and it has been that way a long time, so that would probably explain why she didn't meet up with them again.