I thought the insight into Koyuki’s past was interesting—unlike the usual melodramatic traumas that explain a manga character’s changed personality, with Koyuki it was more like things tend to happen in real life: a kid goes along doing what it seems like everyone else is doing, and because of factors that aren’t even bad ones (being a high-achiever, having a dad who’s a teacher), they end up locked into behavior and thought patterns where they don’t feel like they can be their real selves.
And they can’t even talk to anybody about it. What’s Koyuki going to say when her dad asks her what’s wrong—“Well, Dad, I feel alienated from everybody else as well as from myself because my grades are too high and because you’re a teacher”?
It’s times like that when a person needs a friend. An exceptionally warm, understanding, and affectionate friend.