Bobby Harris and Scott Tierden are Lightsburg High's school bullies, "two real hateful guys who would have been our class clowns if they'd been funny" (4.44). Basically, their primary mission in life is to seek out people who already have enough pain in their lives and rub salt in their wounds while they laugh about it.
Messing with the Madmen in particular is their favorite pastime. When Squid's mom committed suicide, they were at school the next day making jokes about it. They constantly harass Paul for being gay, target Marti for being flat-chested, and make fun of Karl's dead dad. Real classy, huh?
Aside from being just plain cruel, they're also practical jokers of the non-funny variety. "Their idea of funny was to bring a whoopee cushion to a choir concert […] or go into Philbin's, order a banana split, and turn it upside down on the counter with a two-cent tip under it" (7.110), Karl describes. And of course, there's their nightly ritual of splashing mud on the McDonald's front window so Karl has to stay late cleaning it. (Or so they think—he actually is wise to them and saves the windows for last.)