Sonny and the narrator are almost textbook foils. Sonny is impetuous and passionate. He's an artist who loves living in the world of music and musicians. The narrator is rational and logical. He lives the kind of life he's "supposed" to live by being a teacher, husband, and father. We're not saying that one is preferable to the other; we're just saying that the two characters' differences help characterize them because we are able to look at one in relation to the other.