Much like The Whammer, Bump is a guy you shouldn't mess with. He's big, strong, famous, and rich. But Roy can't help himself: Bump has everything that Roy wants, including a gorgeous girlfriend.
Bump is a real jerk; he plays awful pranks on Pop and Roy, and is less than respectful toward Memo. He seems to be somehow immune to her charms and denies the rumors Memo spreads about them getting married. However, his crimes probably aren't worth the death penalty, which is what he gets in the novel.
It's Bump's competition with Roy that does him in. Despite his baseball talents, he's lazy. As the team's star, that seems to set the pace for the other players, who turn into a kind of Bad News Bears version of a baseball team. One Roy unleashes his magic bat, Bump takes notice and starts to play hard. He hustles so hard that, in a freak accident, he bangs into the outfield wall while going after a pitch and gets seriously injured. He dies a few days later.
We can see Bump as a stand-in for what Roy could have been: a professional player with a hugely successful career.