Billy's grandpa plays the important role of mentor, as well as catalyst for much of the action. He's always got his hands in some sort of scheme. Let's take a look at what he does:
And don't forget that he teaches Billy how to catch a raccoon and follows him into a snowstorm at the hunting competition. He's Billy's biggest cheerleader—even more so than his mom or dad. When Billy starts to think that maybe he should stop trying to chop down the biggest stinkin' tree in the forest just to get a wee little raccoon, his grandfather roars: "Give up! [… ]. Now I don't want to hear you say that. No, sir, that's the last thing I want to hear. Don't ever start anything you can't finish" (9.7).
Guess we know where Billy gets his determination from, right?
But Grandpa doesn't just lecture Billy. He actually has some useful advice to dispense, along with all the sputtering about not giving up. Like, he says that he's going to show Billy "how to keep that coon in the tree" (9.11). Now that's some useful knowledge right there.
In any case, you get the sense that Grandpa, like the rest of Billy's family, is really invested in his success—not just at winning competitions and making money, but actually becoming a mature, grow-up man.
Although we have to say Grandpa isn't always the best model. He's got this little habit of coming up with "deals," although naïve little Billy doesn't seem to care: "I didn't care how many deals my grandpa cooked up. He was still the best grandpa in the whole wide world" (14.16-22).
And it's Grandpa who gets him all tangled up with the Pritchard boys' bet. Billy doesn't want to have anything to do with it, but Grandpa won't let the bet go—and it leads straight to Rubin's death.
So, grandpa may be the oldest character in the book, but he also seems like the least mature. He even realizes it himself, when the whole thing is over: "I guess when a man gets old he doesn't think straight. I shouldn't have let those boys get under my skin" (14.12). If this whole book is really about growing up, then Grandpa shows Billy how not to be. Sure, he's there to provide unconditional love and support—but he also shows us that, sometimes, a twelve-year-old boy can be wiser than a grandfather.