The development of Miranda and Sal as characters is central to When You Reach Me. Miranda relies on Sal as her best friend, and the two are extremely close. We can infer that Sal is reliable and intelligent; like Miranda, his parents trust him to be out alone and navigate the city. He and Miranda do everything together in the beginning of the story, showing their dependence on each other.

Unlike Miranda, however, Sal tends to be reserved and doesn't openly share his feelings. After Marcus punches him, he refuses to confide in Miranda and instead, she says, "went straight to his apartment and closed the door on me." As the story progresses, he ignores Miranda instead of telling her that he'd like to find new friends in addition to her. Miranda tries to get him to explain himself and asks if he even still wants to be friends, but Sal "glared at his feet and said no, he guess he didn't for a while." He cuts her off without explaining himself, because in his mind, that's easier.

Sal could also be considered fearful or timid because he avoids problems instead of confronting them. He runs away when Marcus tries to apologize for punching him. When he runs into the street, he is saved from being hit by a car by the Laughing Man, who is killed. This dramatic turn of events might also be considered a turning point for Sal's character.

Later on, Sal explains directly to Miranda that he wanted to make new friends and not rely solely on her friendship. Sal's character development is evident in this moment: he is aware of the negative effect of his actions and has become more aware as a result. It is important to consider this evolution when analyzing Sal's character traits.