...I figured I'd give a little insight into what's going on in Peri's head.
She's got a standing relationship (read: unrequited torrid and hopeless Fiona-style drama-queen Twoo Wuuuv, sorta-kinda) with an (NPC) Fiona knight. She spends a lot of time mooning around over him, as I've shown in various posts. She is, unfortunately, convincing herself at this point that he's done all these awful things to her to test that her love is true.
Being Fiona, of course, she doesn't see a reason to cut out her social life. So, she wound up with two very different sorts of involvements with both Ash and Tommy Lee. Being Peri, she isn't all that good at doing this sort of thing without at least some emotional entanglement, though hers seems to be a little more open and casual than the average fifteen-year-old. She does care about them both in different ways (thanks to that lovely Fiona penchant for romance-at-the-speed-of-light), but there's still the Brandon Issue to resolve.
She is still hoping the Brandon Issue will resolve for the best (which to her, translates as him declaring undying love for her and giving up his wicked ways to bask in the glory of her eternal blah blah blah... fill in with melodrama here), but as time goes on, she finds it harder and harder to believe in it.
Her bounciness is not an act, precisely, but it is, sometimes, forced on herself. There are days when she would prefer to lie in bed and weep over her poor unrequited tragic yadda yadda, but she doesn't let herself. She tries to find something to be happy about in every day of life, which is why she sometimes gets obsessive about little things like candy or her skateboard - sometimes those are the things that let her smile through it all. It's not quite as shallow as it seems, at times. She also is fiercely loyal to people she considers friends, and really does care about them a lot.