While I can't fulfill this exact assignment for you, I can help you think about how to approach this question. If you were to describe Sonny, you would want to concentrate on his motivations, upbringing, interests, and relationships. In your work, think about how Sonny might describe himself. He would likely start with his intense interest in jazz, including the music of Charlie Parker, or "Bird." In the last scene, his brother, the narrator, describes how liberating Sonny finds playing the piano. In your assignment, you can describe Sonny's intense attraction to music and the release he finds in music.
You could also consider how he might describe his upbringing in Harlem. There is a scene in which Sonny speaks to the narrator about how he wants to escape from Harlem.
You might also think about Sonny's family relationships. What do you think Sonny feels about his older brother? He wants a connection with his older brother, but they struggle to understand each other. All of these aspects of Sonny's character can be in your assignment.