One night at Harpo's bar, Sofia arrives with a new boyfriend, a boxer named Buster Broadnax. Mr. ____ asks Sofia where her children are, and Sofia replies that they're "at home," meaning with her sisters. Sofia implies that Mr. ____ does a bad job of taking care of his own children, other than Harpo. Mr. ____'s two unnamed girls live elsewhere, and have had many children, and Bub has been in and out of prison.
Mr. ____ seems to want to imply, somehow, that Sofia is being a bad mother to her children. But of course this is the height of hypocrisy, for Mr. ____ has done absolutely nothing to raise his own children, and all of them, with the exception of Harpo, have either run away or gotten themselves thrown in jail.
Celie realizes that Sofia has started a new life with Buster. Celie feels that her own "new" life began when she (Celie) met Shug. Harpo comes up to Sofia, who explains that she has come to the bar, with Buster, to see Shug sing. Sofia compliments the bar and Harpo's efforts in running it.
Buster, a quiet character in the novel, is a force for good. Although he is a boxer, renowned for his fighting, he mostly spends the novel's climactic scenes finding peaceful solutions for himself and for those around him.
Harpo asks Buster if he can dance with Sofia, and Buster agrees. Harpo and Sofia dance across the floor at the center of the bar, until Harpo's new girlfriend, Squeak, who is small and quiet, comes over and asks to cut in. When Harpo doesn't let her, saying he wants to dance with his lawful wife (Sofia), Squeak gets angry and slaps Sofia.
It is unclear exactly what Harpo's motive is here. He probably does not want to rekindle a romantic relationship with Sofia, but it appears he wants to assert the power he still has over her, as her legal husband.
Sofia, however, as Celie describes, does not deal in slaps, but rather punches Squeak in the face, knocking out two of Squeak's teeth and bloodying her. Harpo clings to Sofia for a moment, after she has punched Squeak, then goes to help his current woman (whom he refers to as his wife, although their wedding has not been described, and their marriage might not be formal). After this fight, Sofia and Buster leave the bar.
Sofia is far too powerful for Squeak. In many ways, Squeak is Celie's foil in the novel. Squeak also undergoes a journey of self-discovery—she learns she has a beautiful singing voice—and she escapes her controlling husband (in this case, Harpo) by the novel's end.