August Wilson tells us all about his protagonist in his stage directions:
Boy Willie is thirty years old. He has an infectious grin and a boyishness that is apt for his name. He is brash and impulsive, talkative and somewhat crude in speech and manner. (1.1.7)
Basically, Boy Willie is a whole lot to handle. He's loud, stubborn, and not afraid to fight for what he wants. Boy Willie does whatever he pleases despite the consequences. This includes breaking the law. He says that he only follows the laws that he thinks are right. This sort of attitude is part of what landed him on Parchman Farm, a prison-turned-plantation in his home state of Mississippi. Of course, many of the laws at the time were directly biased against African Americans, so it's pretty understandable why Boy Willie might not have much respect for the legal system.
More than any of the other characters, Boy Willie is outspoken when it comes to issues of race. He claims to live "at the top of life," meaning that he sees himself as equal to the white man (1.5.52). His sister, Berniece, however, thinks he's all talk. In her mind, just because he goes around saying that he's equal it doesn't mean he is. Though, Boy Willie accuses her of living on "the bottom of life," she sees herself as having a more realistic view of race relations (1.5.52).
Though he is full of bluster and bravado, Boy Willie isn't just all talk. He's also a man of action. In fact, his quest to make a mark on the world drives the action of the whole play. He's determined to buy Sutter's land, where his family was once enslaved. In order to do this, Boy Willie has decided that he must sell the family's historic piano. The piano is in many ways symbolic of his family's struggles over the years (check out "Symbols, Imagery, Allegory"), and Berniece thinks that selling it would be the same thing as selling their souls.
Boy Willie, however, has a much different opinion than his sister. He thinks that selling the piano is the best way to build on what has been left them. He feels like the piano isn't doing any good sitting in Berniece's living room, hardly ever being played. Wouldn't it be more of an honor to their ancestors if the piano was used to buy the land where they were once enslaved? Wouldn't that truly bring them justice?
It's pretty easy to accuse Boy Willie of being insensitive, because, well, he is. The playwright even calls him "crude" (1.1.7). However, he really does seem to think that he's doing what is best for the memory of his ancestors. He doesn't see himself as selling them out at all. Throughout the play, he shows that he values the memories of those who came before him. This is probably best shown when he we him shares the family history with his niece, Maretha. While Berniece tries to shield her daughter from the past, because she thinks it will be too painful.
In the end, Boy Willie lets Berniece keep the piano. It takes a whole lot to convince him though. He doesn't let up on it until his sister uses the piano to call on the spirits of their ancestors to banish Sutter's ghost. This magical moment finally convinces Boy Willie that family heirloom has a value that it deeper than money or land. As Berniece argued earlier in the play, the piano truly is the soul of his family.
Of course, it's not like Boy Willie totally loses the debate he's been having with his sister. In a way, she's come around to his point of view as well. Instead of letting the piano sit unused and thus burying the family history, Bernice now sees that she must embrace the past. Boy Willie reminds her of this as he exits the stage, saying, "Hey Berniece…if you and Maretha don't keep playing that piano…ain't no telling…me and Sutter both liable to be back" (1.5.209).