John “Jack” Baker III is a 13-year-old boy and the protagonist and narrator of the book. He is the only son of parents Elaine Gallagher Baker and Captain John Baker Jr. Jack deeply loved his mother, and the grief and regret he feels over his death is the challenge Jack grapples with throughout the book. His grief and loneliness are intensified by the lack of a close relationship with his father, who was away fighting in World War II for the past four years of Jack’s life. John only returns in time for Elaine’s funeral and subsequently relocates Jack to a boarding school in Maine, far removed from everything Jack has ever known growing up in Kansas.

Jack is curious, adventurous, and competitive. Before Elaine’s death, Jack participated in numerous sports and competitive events back home, even performing well in them—he remembers placing second in a soapbox race in which he built his own car. He was also a member of Boy Scouts and is used to the outdoors and physical exertion. Jack’s curiosity is seen in how he learns the names of all the stars and constellations at a young age and later his unhesitating accompaniment of Early on the latter’s quest.