First of all, thank you, Sonata! How didn't I think about that? And Mimomari sure is growing into a nice mistress.
Back to the plot, Yuuko is the protagonist, but there might be a worse price to pay for that than dissapearing.
Spoiler for Episode 21:
Like becoming aizendad's personal battery. If the guy was capable of creating a center nova he probably has the means to catch one in the act. Wouldn't it be horrible if the next episode is all about the girls coming to terms with Yuuko's dissapearance but in the end they find out NOPE, she's chained up in some weird sugestive machinery all the way back in Cheri's place, probably in a truckload of pain. Talk about mixed feelings. Now Cherry is totally gonna blow his dad's face off in one punch. Nagisa could do it too, I'm not demanding.
On another note, I think this episode did a pretty good job showing why Yuuko is worthy of the position, as cliched as "she tries the hardest" might sound, but that really is the case, she has wanted it from the beginning harder than anyone and sure has busted her rear and soul off to do it, and yet, this girl just seems to aim higher and higher. Especially liked that scene in the turbine (Nagisa's cute fangirl outburst included). Too bad those efforts could easily go to waste now.
Oh, and... ARE YOU A FREAKIN MORON, MII-CHAN?! Not only did you get manipulated by Aizendad TWICE, but your big idea was to forcibly take Acchan back from the idol zone!?
-Good to see things were just as gay back in Acchan's times.
-Did anyone else thought "OH CRAP, IT'S HAPPENING!" when s4 glitched out?
-Fourth condition to be a center nova: Dual wield like a boss