Whipper, Dobbie, and Lewis
These three "wise guys" (14.7) give Adam a hard time on one of his first stops along his bike ride. They're pretty much the textbook definition of bullies. Throughout Chapter 14 they throw things at Adam, ask him a lot of mean questions, and talk about stealing his stuff. On top of that, they think they're pretty cool: "They toss the popcorn in the air and catch it in their mouths as if they're on stage and expect people to applaud" (14.2), and they brag about how they used to hitchhike (which, by the way, was more common and probably safer then than it is now). In the end, these guys actually knock Adam off his bike, hurting him, but also moving him along on his journey (because he ends up getting a pity ride from some nice strangers).
These three boys have a way of making Adam feel really bad about himself, and they bring out the self-defined coward in him. But, Adam notices, Whipper is a pretty typical looking teenager, besides the scar on his forehead (14.8, 36). So the bullies in life are no different than the rest of us.
One cool thing Robert Cormier does with these guys is give us another one of his very subtle foreshadowings. Adam mentions that these are the kinds of bullies you see "everywhere in the world, in schools and offices, in theaters and factories, in stores and hospitals" (14.7). Hospitals? Yikes.