Quistis is one of those more interesting and involved characters. She's also one of those "oppressed" characters as well. During the course of the game you watch her as she struggles her inner battles to overcome her weaknesses and find her true self.
First thing you'll notice is that she's the exact opposite of Rinoa Heartilly in almost every way right down to the looks. Quistis is calm and serious where as Rinoa is outgoing and spirited. Rinoa can easily voice her mind where as Quistis cannot and most importantly, Rinoa shows her feelings towards Squall openly and persues him whereas Quistis remainind in the background and lost any chance she could've had with Squall.
Quistis strives to gain the attentions of the one person she really thinks she can confide in, Squall. Squall on the other hand, is cold and secretive leading Quistis into even more depressed states after he "blows her off" while she spills her heart out to him after the SeeD graduation ball.
That leads you to wonder why she was trying to talk to Squall in the first place. Well, in the beginning you assume that she has a thing for her student and that there's a discrete romance going on. I swear, there is! Later, Quistis claims that those "feelings" were just her sisterly protective instinct from when they were all children. (Yeah, right)
Quistis is calm and reserved and very rational when it comes to making tough, life-threatening choices. She carefully thinks through everything and normally doesn't jump to conclusions as Rinoa does. She's very logical and very smart to be a SeeD at fifteen which makes me look up to her. She's one of those real role-model type people.
But then again, Quistis isn't perfect. If Square had made her perfect, I don't think this shrine would be here today. She's smart of course, but she's like all people. She has insecurities and needs someone to comfort her. Makes you feel really bad because the only person she's looking for comforting from is Squall and he's kind of a block head during the first part of the game and doesn't take kindly to Quistis' hints.
In any case, Quistis is a one of a kind character. I really loved how Square gave her a good, meaningful (though really complex) personality.