There are only two major characters in "Sonny's Blues," the narrator, and Sonny himself. Both are round characters.

A round character is a character with depth and complexity. There are various ways of testing whether a character is flat or round. One is to ask whether the character's personality can adequately be summed up in a single sentence. A flat character's can, while a round character's cannot. Another is to think about whether you can predict what a character will say and do in any given situation; round characters have the capacity to surprise you.

Sonny surprises the narrator during the course of "Sonny's Blues." The narrator knew nothing of his brother's struggle to escape from the drug culture of Harlem, or of the true relationship between music and drugs in Sonny's life. These revelations cause the narrator to change his own attitude and conduct.

Sonny and the narrator are dynamic characters as well as round characters. These two concepts are related, but not identical. Dynamic characters change over the course of the narrative. Round characters demonstrate the complexity and capacity for thought that suggests they are capable of change, whether they actually change or not.