Sidney McCauley is the heir aparent of the very leadership he seems to disagree with, having joined the group during Allie 's exodus. He is Allie's foil through most of the novel, growing from someone who threatens her leadership to someone who earns her trust. Sidney constantly questions Allie's judgment even after he is proved wrong and vouches his belief that humanity is not a destination that must be lifelessly pursued but a choice that is made at every crossroads. His friction with Allie makes them both more able and responsive as the story progresses. He is paired with Jameson as his second.
Sidney is noted for his eventual persuasion of Allie to leave the code behind and come into the new world she had hoped to build, but not live in. Sidney inscribes the new code onto his late predecessor's grave stone.