The main character of "Sonny's Blues" is the narrator, who tries to come to terms with what has happened to his brother. At the outset of the story, after he hears that his brother has been arrested for selling heroin, the narrator says, "I couldn't believe it: but what I mean by that is that I couldn't find any room for it anywhere inside me." The story revolves around the narrator's attempt to make sense of what has happened to his brother by going back into their past and then watching his brother play music, an experience that helps the narrator understand who his brother is and what he values.
If Sonny had told the story, it would have been very different. Instead of Sonny's brother trying to figure out Sonny, Sonny could have told his own tale of what happened. Sonny would've explained what drove him to do drugs and how the narrator, his brother, and his family had not understood him. He would have perhaps included other reasons why he turned to drugs. "Sonny's Blues" involves one character trying to unravel the psychological mysteries of another character; if Sonny had told the story, it would have involved less psychological mystery and would have been a more straightforward narrative.