This is the police officer who is supposed to find Margo Roth Spiegelman, but isn't really trying. He calls Margo Roth Spiegelman a balloon (because he talks in metaphors like he lives in Twin Peaks or something), saying that "you can see all of the balloons, but you cannot see any one balloon" (2.3.43) and eventually "she'd deflate and float back to Jefferson Park" (2.3.45). Basically, he's saying that she isn't that special, and since she's an adult now, according to the law, it's up to her to make her own decisions and come back if she wants.
These two are trusting therapists who occasionally give Quentin some insight into the human psyche, like when they say, "It's so hard for anyone to show us how we look, and so hard for us to show anyone how we feel" (2.15.18). This makes Quentin realize that Margo Roth Spiegelman might be having an identity crisis. He says that because his parents are therapists, it "means that [he is] really goddamned well-adjusted" (Prologue.21). But how well-adjusted is someone who obsesses over a girl he's hung out with twice, enough to chase her across the country? Over to you, Shmoopers.
We meet a few classmates from Quentin's high school. Cassie Hiney is "perfectly nice and pleasant and cute" (1.1.5), which are the most important qualities for a high-school female, we guess, despite being named after a butt. Chuck Parson is a bully who calls other boys "faggot" (1.1.32), and loses an eyebrow. Jase Worthington is the boy who cheats on Margo Roth Spiegelman, has a tiny penis, and eventually becomes friends with Ben. Dr. Holden, English teacher, helps Quentin analyze "Song of Myself."
Gus was a senior when Margo Roth Spiegelman was a Margo Roth Spiegelfreshman. He is security guard at the SunTrust building and does "Urban exploring" (2.16.48), which doesn't involve stalking Keith Urban.
Robert Joyner is the dead guy who kind of haunts Margo Roth Spiegelman for a while, though not literally. She and Quentin find him in the park when they're nine, and it gets Margo thinking that she doesn't want to die in her hometown—she wants to get out and see the world.