Bunny is Jack's best friend (not to mention his foil—check out the "Character Roles" section for more on that). She's also got one ironic name, because she's anything but Bunny like—except for her size.
See, she's a very small girl who can "run full speed under her dining room table without ducking" (5.1). Wow. Either that's a tall dining room table, or she really is a wee little thing.
There's a big personality in this little package, though: she's "tough, smart, and daring," and Jack thinks she's "better than any guy" (5.3). (Gee, um, thanks?) As you can probably guess, she's kind of a tomboy, too. She's active and hands-on, constantly shoving, poking, and throwing things. And people.
Her dad is the owner of the town's funeral parlor, so Bunny basically "grew up in a house full of dead people" (5.3). Sounds kind of gross, right? Sure, if you're not used to it—but so is peanut butter, if you're not used to it. Bunny doesn't think it's gross, and she's not afraid of anything. She even tries to get Jack to overcome some of his fears, like when she physically makes him touch a dead body. (Maybe not the most sensitive way to do it.)
Bunny has a good sense of humor and—-no surprise—a morbid one. She tells lots of dead people jokes ("Look alive, you bunch of stiffs" [5.2]), and gets a huge kick out of Jack's squeamishness. At one point she slaps into his hand a set of dentures that have just recently been inside the corpse of Mrs. Slater (5.34).
Do these practical jokes remind you of anyone? Just like Jack's dad, Bunny has kind of a mean streak. She throws a ball at Jack's head (5.16)—on purpose—and she continually gets angry at Jack because he's been dumb enough to get himself grounded. She also doesn't like anyone to disagree with her, and when she gets angry—well, "she seemed to compress down into an even smaller version of herself so that she looked like an angry tree stump with short stubby branches for arms" (17.3).
So, here's our question: what does Jack get out of this relationship? Does she represent the tough, no-nonsense, and no-nosebleed kid that Jack would like to be? Or, like Jack's dad, is she just an immature bully taking advantage of someone who she sees as weak?