Loudmouth Good Guy
Angelo Maggio (Frank Sinatra) is Prewitt's loudmouth best friend. He's a good guy—in fact, one of the only guys who overtly sticks up for Prewitt—but his vibrantly cocky attitude gets him into trouble again and again. And again.
But his benevolent attitude is demonstrated in this little exchange with another soldier, Corporal Buckley. The "good guy" in question is Prewitt:
MAGGIO: I just hate to see a good guy get it in the gut.
BUCKLEY: You better get used to it, kid.
Maggio drags Prewitt to the New Congress Club, a social club that seems a lot like a brothel (because it was one in the book), and ends up starting a dumb fight with Sgt. Judson for playing the piano too loudly and badly. This little tiff isn't the last interaction between them: after Judson checks out a photo of Maggio's sister at a bar the soldiers hang out in, and reacts inappropriately, they almost get into a knife fight.
It shows how Maggio is protective of his family and his loved ones—including his best friend, Prewitt. Despite Maggio's impulsiveness and his flaws, he really does care about other people—and is willing to take part in Prewitt's sufferings for no other reason than personal affection and maybe a sense of fairness. But after the failed knife-fight, there's worse to come.
Human Punching Bag
One night Maggio is excited to have some time off—he's ready to go out on the town. But at the last second, he gets pulled into guard duty. He decides to abandon his post and go into town anyway, where he gets extremely drunk.
This is an idiotic move. It leads to a court-martial for going AWOL and lands him in the stockade under Judson's watch. Judson proceeds to beat him viciously over and over again. When Maggio finally escapes, he ends up dying from his injuries—leaving Prewitt to seek revenge.
That's Maggio, simultaneously kindly and too impulsive to preserve himself. He seems a little desperate, in a way—especially in the scene right before he's arrested. He runs away from guard duty in order to get way too drunk, openly challenging MPs to arrest him.
Even though he's one of the good guys, and is totally generous and kind in relation to Prewitt, there's something a little self-destructive about him. He picks fights with the exact wrong people—like Sgt. Judson—and goes AWOL in a way almost calculated to get him trouble. But in the end, his loyalty and propensity for friendship redeem him and make Prewitt willing to risk his own life for the sake of Maggio's memory.