Calm, collected and mature, Quistis is the young instructor whose class Selphie is placed in upon transfer to Balamb Garden. Not too surprisingly, the first time Squall sees Selphie she is running late for said class; still, the two girls get to spend more time together after Quistis shows up at Timber during the new SeeDs' first mission, subsequently retreating to Galbadia Garden with the rest of the party.
There isn't much visible interaction between these two throughout the game; however, it can be said that they are complete opposites, with Quistis being shy, reserved and hesitant, much unlike Selphie: if some may argue Selphie acts younger than her age, Quistis, on the other hand, resembles a woman a dozen years older. Still, this doesn't seem to be a problem between them: on her website, Selphie disagrees with Quistis's self-criticism of being too serious, saying that she "really looks up to her", and the two of them are seen together in the ending sequence, first stepping away from Irvine during Selphie's little jealousy fit and then saving Zell from choking on hot dog.
Overall, Selphie has great respect for Quistis, not unlike the kind she has for Squall. Contrary to what could be expected, their relationship is not too formal; they have several friendly moments and are probably a good influence on each other, Selphie most likely encouraging Quistis to loosen up a bit and the latter serving as an inspiration to the younger woman.