...How did the other two characters actually develop more than this, though? You learn more about their backgrounds and what makes them who they are, but fundamentally they don't change much.
As far as I can see, the Alto at the beginning of the series would still make the same decisions that the Alto at the end of the series would make. He still chose the sky over the girl (or his old life).
Sheryl had a bit more development, but she also had far more screen time toward the end of the series, not to mention impending death to move things along.
All in all, Ranka may have suffered from under-development (though as pointed out by FV, not as dramatically as some would make it out to be, but she didn't fare that much worse development-wise than the other two leads, and arguably far better than Alto.