Nice name, right? You expect a guy called Whambold to knock 'em out of the park, and that's what Walter "The Whammer" does. He's the big oaf on the train who doesn't talk much, but is sure enough of himself to take a bet that Roy can't strike him out. Most readers have understood The Whammer to be based on the great slugger Babe Ruth.
The Whammer's one of our first examples of pride going before a fall. When Roy strikes him out, he's basically out of the novel. The young phenom bumps off the older champ. We can understand The Whammer as a sort of a foreshadowing for what's to come with Roy. At the end of the novel when Roy faces the young pitching newcomer from farm country who strikes him out, the pattern repeats. This is making Shmoop so depressed that we have to watch the happier movie ending again this minute.