Rinoa likes to pretend she's perfectly innocent, all gentle smiles and angel wings and child-like curiosity. She pretends she's strong, too, and she pretends she's happy all of the time, but no one other than Quistis has seemed to catch on to that bit, because her smiles are oddly convincing. Although Quistis can see through them, because she knows exactly what it's like to put up oddly convincing smiles and answer people with almost-real-yet-not-quite-laughter. Her emotions are easy to tell apart from one another, though – when she's sad, her face creases just a little in certain places, and her eyes, pretty as they are, get a little darker and a little moist, and Quistis can't help but feel upset as well when she sees her like that.
A part of Quistis is jealous of her, because she knows she's not that honest or innocent or happy, even if the happiness isn't really there all of the time.
She knows Rinoa's really not as innocent and honest as she lets on, though.
She figures that out when Rinoa slips into bed with her and traces her finger down the center of her stomach, around her bellybutton and over her abs, to wake her up in the morning, and her almost-there-smile is actually a bright-sunshine-smile, and Rinoa proves to her that she's really not as innocent and child-like as she might look.