A librarian at the Newberry Library in Chicago who suffers from Chrono-Impairment, a rare genetic disorder causing involuntary time travel. At the start of the book, he is a lonely twenty-eight-year-old unaware of the extensive history he has already shared with Clare during his future travels.

An art student specializing in paper sculpture who has known Henry since she was six years old due to his visits from the future. Twenty years old at the start, she has been waiting her entire life to finally meet the "real" Henry in the present time.

A lawyer and the boyfriend of Clare's roommate, Charisse, who is fiercely protective of Clare. He harbors a secret, unrequited love for Clare but suppresses it to remain a loyal friend to her and a committed partner to Charisse.

Clare's bubbly and supportive best friend and roommate in Chicago who acts as a grounding force in Clare's life. At the start of the book, she is dating Gomez and remains unaware of the true nature of Henry's condition.

A geneticist and researcher whom Henry eventually seeks out for assistance with his condition. Although he appears later in person, he is mentioned early on by Clare as a figure representing scientific hope for managing Henry's time travel.