From the very beginning Yuki is classified as childish and greatly underestimated as a character. But there are hints all over the movie that let the viewer know she's not as crazy or dumb as people think. Although she may seem delusional, none of her actions put any of the others at risk, unlike Miki who almost got them killed. And in that very situation Yuki saves Miki's life. Yuki is always smiling and pretending to talk to students and teachers that aren't there, but when things get serious she pulls through. Yuki is my favorite character. :D