Sonny may be considered the protagonist of "Sonny's Blues" because of the conflict he faces and the emotional response he invokes in the reader.
By examining the title, readers are reminded that this is Sonny's story. Though the narrator is certainly central to conveying Sonny's struggles (as well as his own), Sonny's plight is the one readers become most emotionally invested in. Sonny's conflict is with his setting; Harlem presents a menacing background that seems to ensnare Sonny in various trials throughout his life. His dream is to become a jazz musician, but not even his brother takes this dream seriously.
Prior to the events of the story, Sonny endured the death of both parents and then the physical loss of his brother when he left to serve in the military. He attempted to share his personal struggles with his brother, begging to get out of Harlem and insisting that he didn't learn anything in school that he'd want Sonny to learn. The narrator insists that Sonny remain in Harlem, and eventually, the culture enveloped him. He is arrested for selling and using heroin, which is a fate that an old friend believes Sonny is destined to repeat over and over.
Following the death of the narrator's baby daughter, Sonny and his brother begin a complex journey to construct a new relationship based on acceptance and empathy. The narrator thus finally begins to appreciate the incredible significance of music in Sonny's life and opens himself up to allowing Sonny's music to bridge the chasms of pain the brothers have endured.