Bob is Cherry's boyfriend, and he's a complete dirtbag. In a novel full of mostly likable characters, this guy is completely hate-inspiring.
He's the hard-drinking ringleader of one of the gangs of Socials terrorizing Greasers. Pony and Johnny are pretty sure Bob is the guy who kept his rings on while using Johnny's face as a punching bag:
Johnny was breathing heavily and I noticed he was staring at the Soc's hand. He was wearing three heavy rings. (3.49)
Yup: he's one sadistic bully. He's also the guy who orders David to "give [Pony] a bath" in the fountain in the park.
We aren't sure exactly what happens while Pony is being drowned, but we imagine that Bob approaches Johnny. How else would Johnny get close enough to stab him? Interestingly, Johnny says he killed Bob to stop David from drowning Pony. So maybe he went after David with the knife and stabbed Bob when Bob tried to stop him. These are things we can't know. How do you picture the scene, though?
Most readers have difficulty mustering sympathy for Bob, even though he gets killed in the novel. We aren't shown any of his good qualities. Cherry's doesn't even offer any justification for her love for him, other than that he's good-looking and charismatic. If Cherry had told us about Bob's kind acts, or something like that, we might have begun to see what she sees in him, other than his good looks, money, and popularity.
Randy doesn't try to justify why he cared for Bob, but he does offer some explanations for Bob's behavior:
"They [Bob's parents] spoiled him rotten. He kept trying to make someone tell him "No" and they never did. […] That was what he wanted. […] To have somebody lay down the law, set the limits, give him something really solid to stand on." (7.106)
So, basically, Randy believes that Bob was so out of control because he didn't have enough discipline or boundaries at home. Do you think this sounds realistic? We might consider that many kids with non-authoritarian parents don't turn into sadistic bullies. But Randy takes his theory even further. He says,
"If his old man had belted him—just once, he might still be alive." (7.106)
Now Randy almost sounds like he's repeating something he heard another adult say: this "spare the rod, spoil the child" line seems bizarrely middle-aged. Pony and his brothers would certainly argue that corporal punishment isn't necessary to raise a kind child. Randy seems to be suggesting that if Bob's father had been more violent toward him then Bob, in turn, would have been less violent. How do you feel about this? In the end, Bob remains a mystery to the readers, and probably to Pony as well.