Sonny is the story's truly tragic character. It actually hurts to read about his struggles. Take a look at this passage as he tries to explain his inner turmoil to the narrator:
I was all by myself at the bottom of something, stinking and sweating and crying and shaking, and I smelled it, you know? My stink, and I thought I'd die if I couldn't get away from it and yet, all the same, I knew that everything I was doing was just locking me in with it. (218)
We can actually feel Sonny's suffering throughout the story. On some levels he represents hopelessness, and most readers can probably feel for this man who saw no other way out of his troubled life than to turn to drugs.
But this same fact about Sonny's life might also make many readers angry. It seems like Sonny and some of his friends try to justify his drug use, and a lot of readers probably feel that there is no justification for becoming a heroin addict and causing other people so much pain. We might sympathize with Sonny's inability to escape Harlem, but we might also feel that he had other options than to start taking drugs. After all, his brother grew up in the same environment and managed to start a family, have a career, and create a home. Maybe Sonny suffers most from making really bad life decisions.
Yet another possibility: perhaps Sonny is the story's hero. It all depends on how we read the ending, but if we believe Sonny has really conquered his drug addiction and is starting to turn his life around, we could see something heroic in this. After all, recovering from heroin addiction is a pretty monumental accomplishment and something we can applaud.
Here's the thing about Sonny as a literary character: he could be one or all of these things. He could be the tragic figure, the heroic figure, or the antihero. Or maybe he's a little bit of each of these. From a literary analysis standpoint, Sonny is such an important character not just because he is central to the narrative but also because he shows how one character can fulfill many roles depending on how we interpret the story.