Although Selphie is an overall optimistic, happy-go-lucky person, what truly allows us to see beyond the surface are the times she feels blue or depressed. These moments make her fragility show and trigger a serious side of her that is otherwise invisible throughout the game.
Selphie goes through a particularly bad phase from the moment she leaves the D-District Prison in Disc 2 to the end of the second battle against Edea. This is easily explained by a number of factors that, combined, have a crushing effect on her: first, her true home, her dearest Trabia Garden, along with all of the people she grew up with, faces destruction at the hands of Galbadia. Not only was she powerless to stop the missile attack, there may be some guilt involved concerning the mere fact that it happened, too, since the only reason Galbadia even retaliated was the party's failure at assassinating Edea at the end of Disc 1 – at the Missile Base, Selphie behaves almost hysterically, going on a full-blown killing spree than even leads Irvine to comment that she can be "a little out of control" at times; this type of reaction is usually a denial-like human defense against traumatic situations such as hers. A while after, when she is reunited with the rest of the team in Fishermans Horizon, she comes to find that the Quad stage, where she planned to host the Garden Festival, has been destroyed with the crash against the town. At this point, the more important things in Selphie's life are not just slipping through her fingers: they are quite literally falling apart.
In her diary, this is when Selphie mentions feeling blue – and despite Irvine's successful efforts to cheer her up by finding her a way of organizing a concert, the joy is only temporary, and she soon faces the difficult decision to go visit the ruins at Trabia. After gathering all of her courage, she asks Squall to take her there: she sounds uncomfortable and hesitant when approaching him, almost shy (very much unlike her usual self), and it is obvious that, as Quistis comments, she is trying hard to act cheerful. The subsequent scenes in Trabia are extremely important in the definition of Selphie's true character; once there, she is seen running around, visiting her friends' graves (where she can be found making a touching speech), spreading hope and support among the survivors and, in the process, tying all ends together in her mind, striving to find closure. At last, she thinks it possible to pull herself together and decides that things will be okay after she gets her revenge against the sorceress; however, this solace is once again short-lived, for just a few moments later the party regains important memories from their childhood, and the truth is cruel: it is not the evil, distant sorceress Selphie expected that she will be facing, but her beloved Matron...
Selphie has to deal with a lot in an extremely short period of time: she must get used to the idea that Trabia is in ruins, that the Garden Festival she had worked so much for is no more, and that, in the end, she will supposedly be killing the woman who took her in as a child. Given the circumstances, it is truly admirable that she manages to maintain such enviable mental stability, and even more so that she lifts herself up and channels all of her rage into an effective response to the events that have brought her down. The party's presence, of course, plays a prominent role in this, especially Irvine's: it is also in them that she finds the strength to keep the decision of facing Edea to the very end. Once the latter goes back to being their Matron, Selphie undergoes a drastic change: now that things are "back on track", she feels that she can take on anything and anyone, and thus her old self is back, much to Irvine's joy: after all, the two of them are seen together a lot more often from then on, and it is hinted that they have become a support pillar for each other, even if, contrary to Squall and Rinoa's, their more intimate moments are never shown.
Not only do these less than perfect periods serve to show that Selphie does have a serious side to her and that she is more mature and sensitive than she seems, they are also the living proof that she is a strong, selfless individual: one that not only is capable of lifting herself back up, but also finds it natural to give whatever energy she has left to those who need it more.