Generally speaking, when (desire to app someone new) + (boredom with current character) > guilt, a character gets dropped. But to elaborate:
Ema is odd in that she's the character who has the least meaningful connections here, but is also the one that I'm least likely to drop. She takes little to no effort to get into her mental state (I'm... not sure what that says about me), has the most frequently amusing threads, and is just generally the easiest for me to get out. She could definitely survive the dropping of absolutely all of her castmates and... any of her important relationships in camp without becoming unplayable. On the other hand, I wouldn't feel guilty at all about dropping her, buuuut I have absolutely no desire to do it. She's here until exactly the moment I get bored of playing her!
Keiko was on the edge of being dropped for a really really long time, but now I'm in a pretty good place with her! I'd never really had a character who I considered a backburner before, so it took a while to adjust to that, but I actually really enjoy playing her when I get her out. She's here for the long haul. /o/
Zuko was apped on a sort of whim, but I absolutely can't fathom dropping him. He's the one I worry most about being IC with, and who I flail about the most, but also every time I worry that I'm being too dorky canon goes and out-dorks me so it's okay. Anyway, I love playing the adorably whiny socially-awkward short-tempered dweeb, he has a huge awesome cast now, and I think if I had to drop all but one of my characters right now he'd be the one that stays.
Saionji I'm having all kinds of fun with and is one of my favorite characters from one of my favorite series ever, but the short easy answer is that he will be in camp exactly as long as Touga is.
Kururu I just got so I have absolutely no idea, but it's the double-app with
tinaeu that I've always wanted. SO HOPEFULLY WE CAN BE SUPER-RETARDED FOREVER.