Cassidy is Sutter's "beautiful fat girlfriend" (1.1) … who dumps him as soon as she possibly can, because she's actually a cool chick who's got her life together.
She also might be his only real friend. She and volunteer-happy Marcus are a good match, but she misses the fun she had with Sutter. After the break-up, they hang out a lot, and their relationship evolves while Cassidy keeps a sisterly eye on him. Sutter explains: "It's weird. Our relationship is doing this surreal metamorphosis right in front of my eyes. […] Cassidy's […] like some kind of new, mutant, never-before-seen type of friend" (47.22).
Maybe—and we're just throwing this out there—maybe Cassidy is Sutter's first friend who actually stands up to him, and who actually has his best interests at heart.
In fact, Cassidy seems to have everyone's best interests at heart. When Sutter starts dating Aimee, Cassidy takes on a protective role for her, constantly reminding him to treat her right. Even when Aimee cusses her out and slaps her in the face, she stays calm and tells Sutter to take Aimee home. In the end, though, she chooses Marcus's world over Sutter's—and who can blame here? "I'm moving out here to go to school," she say. "Marcus wants me to, but I wasn't sure I wanted to until now. […] It's like a whole new era in my life is unfolding, Sutter" (65.7-18).
Translation: she's growing up. She's moving into the future, away from the now. And that means leaving Sutter behind. Once starts down the path towards adulthood in earnest, she's left Sutter behind.