Dirty Deed is the one white member of the band and the member who plays the piano. When Mr. Jimmy introduces the band members to Bud, he presents Dirty Deed in the following manner:
"... and the palest member of the band, on piano, is Roy 'Dirty Deed' Breed."
What is distinctive about Dirty Deed is that he is the one white member of the group. Later on we discover that this is largely for legal reasons, as during the time blacks were not allowed to own property, so Mr. Calloway always had one white member in the group to allow him to maintain possession of his Cabin. However, it is not just for legal reasons that Dirty Deed is a member of the band - it is clear that he is an accomplished pianist who works incredibly well with the other members of his band. Consider Bud's description of his playing in Chapter 17:
Then Dirty Deed started making the piano sound like it was a kind of drum, for a second it fell right in with the rain pats that the Thug was making, then it took off and made you think of what Niagra Falls must sound like, it sounded like big, bright drops of water splashing up and over, over and up. The drops would fall loud and clear as anything, then before you knew it they were right back into Thug's steady, bouncy beat.
This section of the novel therefore demonstrated Dirty Deed's tremendous ability in playing the piano, but also his ability to play as part of a band - someone who is able to listen to and respond to other members of the band and how they are playing and play in a way that complements their style. Also note the simile that Bud uses to describe his playing by comparing it to Niagra Falls.