Jack Sullivan is the 13-year-old protagonist of the story. He is the de facto leader of the group and is responsible for bringing everyone together in the first book of the series. While he is not the strongest or smartest, he is incredibly brave and always adventurous. He is also the heart of the group, and he will do anything—even recklessly risk his own life—to protect his friends. Perhaps his most important trait is his relentless optimism. Despite the dire situations the group finds themselves in—not to mention the broader, postapocalyptic context—Jack always finds a way to look at things positively. The most prominent example is his eagerness to turn everything into a quest, rather than viewing it as a chore, and his ability to make jokes in life-threatening moments.

Before the Monster Apocalypse, Jack was an orphan who bounced around the foster care system. He never stayed in one place long enough to develop meaningful friendships or feel a sense of home and belonging. This experience frequently informs how he treats others, particularly monsters, as he is able to put himself in their shoes and understand how they feel as outsiders. This is evident in the way he treats Alfred the zombie with care and respect, and in his ability to recognize that the Wormungulous was attacking them because it was scared and confused, rather than evil.