Joe Bigbear is Harlen's older brother. Will wasn't aware that Harlen had a bother until Floyd calls to let him know that Joe is in town and insists that Will come and meet him.
When Will arrives, Joe shakes his hand hard; Will feels like he's trying to find all the bones under his skin. Then he sits him down and offers him a cigar, instructing him on how to prepare and smoke it. Then he regales the gathered people with a tale about how he tried to catch a pig and other stories about his travels.
Joe is a world traveler while Harlan is a community-oriented person. Joe is a big talker who tells stories about his adventures all over the world. Harlan isn't impressed with his brother, and it is obvious that there is some kind of problem between them. When Will brings up that Harlan never told him about his brother, Harlan gets quiet and doesn't say much about it even when they go to a large bridge together with the goal of jumping from it.
One major thing that Joe does which Harlan wouldn't do is egg Will on, wanting him to jump off the bridge despite Will's obvious reservations. It's clear that Harlan wouldn't do this because he keeps putting Joe off and telling him to show them how it's done instead of pressuring Will to be the one to do it. He's willing to push Will into things—like basketball or dating—but not things that will be dangerous or could lead to him being injured.
The other thing he wouldn't do is jump off the bridge. He's not a risk-taker in the same way Joe is. He has a regard for his own life. When Will decides he's going to copy Joe and jump, Harlan just says okay and waits for him to come to his senses. It's clear that Harlan has been up on the bridge before and has failed to jump before. He puts more value on his own life than Joe does on his.
Interestingly, Floyd says that Joe is a good guy and nothing like his brother Harlen. The implication here is that Harlen himself is not a good guy despite Will's friendship with him and the positive impact he appears to have on the community. While Joe and Harlan are both outgoing people, there is something that Floyd prefers about Joe. After Joe leaves, Will says that he never saw him again.