Berniece is one tough cookie. Much like her brother, Boy Willie, she's not afraid to call it like she sees it. She is the strongest voice of femininity in the play, and in many ways represents her family's maternal line. Berniece holds her own against the men of her family and refuses to let anybody tell her that she needs a man to be a whole woman. Her pseudo-boyfriend, Avery, is constantly trying to get her to marry him, but she keeps him at a distance.
Of course, Berniece's choice to not remarry is much more than just a feminist statement. Stage directions tell us that "she is still in mourning for her husband after three years" (1.3.30). It seems that in many ways she is pushing everybody away, because she is still wracked with grief. We see this grief explode at the end of Act 1, when Bernice physically attacks Boy Willie, who she blames for Crawley's death. In Act 2 we see her start to come out of her shell a little bit when she allows herself to be kissed by Lymon and even kisses him back. (Boy, Avery sure would be mad.)
Berniece serves as the antagonist for much of the play, standing in the Boy Willie's way as he tries to sell the family's historic piano. She tells her brother, "Money can't buy what that piano cost. You can't sell your soul for money. It won't go with the buyer. It'll shrivel and shrink" (1.2.158). She feels that it is almost sacrilegious to sell the piano, since so much of their family history is wrapped up in it. Berniece and Boy Willie's father, Boy Charles, even lost his life over the piano. Boy Willie tells his sister that she is wasting the legacy that their father left them, and sees selling it for the land they once were enslaved on as honoring their father's memory.
What makes Berniece's stance on not selling the piano slightly ironic is the fact that she does her best to forget the family history. If it's it so important to preserve that past, then why won't she tell her daughter, Maretha, the family history? Berniece argues that she doesn't want to burden Maretha with the past, but is that really just an excuse to hide from it herself? Berniece even refuses to play on the piano any more. Her mother used to make her play on it all the time when she was a little girl, saying that she could hear the voice of her dead husband when Berniece played. Bernice thinks that the piano is literally haunted by the spirits of her father and other ancestors and thinks it's best that these spirits not be awoken.
In the end, it turns out that Berniece is totally right about this whole haunted piano thing. When the ghost of Sutter attacks, Berniece finally conquers her fears and plays the piano. She calls on the spirits of her ancestors to help banish Sutter's ghost. Berniece finds a spiritual connection through the piano and calls on not only the strength of her ancestors, but on the power of the African American race. By the play's conclusion, she cracks out of the shell that she's been hiding in. She reclaims her womanhood and her heritage.