Here's what August Wilson has to say about Doaker:
He is a tall thin man or forty-seven, with severe features, who has for all intents and purposes retired from the world though he works full-time as a railroad cook. (1.1.2)
Doaker does his best to keep the peace between Berniece and Boy Willie as they argue over the piano. He admits to Wining Boy, though, that he wishes Berniece would let Boy Willie sell it, because it reminds him of their brother's, Boy Charles's, death. Doaker also thinks that the piano is the reason Sutter's Ghost is in the house – he tells Wining Boy that he once saw Sutter's ghost sitting at the piano and that he's heard it play music by itself. Given all this, though, Doaker still stops Boy Willie from taking the piano from the house without Berniece's permission.
Being one of the elder members of the family, Doaker functions is one of the keepers of the family's history. It is from him that we hear the long and bloody history of the piano. Much like his brother, Wining Boy, he functions as a griot, or African storyteller. Doaker passes the oral history of the family on to the next generation.