I didn't find Yoko annoying at all, though people warned me that I might -- I thought she reacted in a way that made sense considering she's a high school student and not equipped in any way for magical unicorns and monsters and things. Anyway, she does get better at existing in a world with magical unicorns etc. etc., so do stick with it if you can. 12K isn't one of my favourite anime evar, but it's good fun, very old-fashioned fantasy YA series in feel.