Ben Starling, self-proclaimed owner of the "World's Largest Balls" (2.16.14), is an "olive-skinned creature" (1.9.9), which is the way a white author describes a racially ambiguous character who could be a minority, but will probably be white in the movie.
He is Quentin's best friend because they're about on the same level of the social totem pole (a.k.a. the bottom). Becca Arrington calls him "Bloody Ben" (1.1.14) because he had a kidney infection, complete with bloody urine (why did he share this at school?), and she spread a rumor it was caused by chronic masturbation.
He thinks this is the reason he can't get a date, although it could also be because he's sexist, saying things like "you should just hit that" (1.1.17), with that being a human female named Margo Roth Spiegelman. He also calls women "honeybunny" (1.1.17), and the last time we heard that was from the psycho who robs the diner in Pulp Fiction.
Ben is the main source of conflict with Quentin, because Ben insists that Margo Roth Spiegelman is a drama queen who just wants attention, and Quentin is mad that Ben has the audacity to make other friends, especially friends who do things like go to prom and do keg stands. As Quentin explains it, he hates Ben for "jump[ing] at the first opportunity to join the fraternity of vapid asshats" (2.14.12).
So why does Quentin even like Ben? He sums it up thusly: "He tried hard" (1.1.9). Also, Ben is a loyal guy, helping Quentin out even if he doesn't believe in the reason for the quest (to find the mythical Margo Roth Spiegelman). Ben is also the only main character whose last name doesn't follow what must be a Florida tradition of requiring three syllables and ending in n: Jacobson, Spiegelman, Arrington, Pemberton, Worthington. And hey, variety is the spice of life.
Ben ends up dating Lacey Pemberton, the only girl at school who could remotely be called one of Margo Roth Spiegelman's actual friends. He tags along on the road trip to find Margo more because of Lacey than because he actually wants to find her.
It's a good thing he comes along, too, because when Quentin chokes under pressure (as he has a tendency to do), Ben takes the wheel. Literally. Quentin almost crashes the car into a cow crossing the road (why did the cow cross the road?), but Ben grabs the steering wheel and guides them to safety. Everyone calls him a hero, but like a fireman just doing his job, he won't accept the title.
So Ben goes from being made fun of as Bloody Ben to having a hot girlfriend, new friends, and a being a life-saving hero. Good for him. One thing about him that doesn't ever change, though, is his disdain for Margo Roth Spiegelman. When they finally find Margo and she's just as rude as she was before she left, he tells Margo, "I like the clues more than I like you" (3.22.36). So not only is Ben a loyal friend; he's always honest.