Oona is an 18-year-old keyboardist who experiences a time slip on the eve of her 19th birthday, suddenly waking up in her 51-year-old body in the year 2015. She discovers she will leap to a different random year of her life every New Year's Eve, completely out of chronological order. Thrust into sudden wealth and unfamiliar eras, she relies on letters written by her past and future selves to understand the rules of her non-linear existence. She harbors a secret desire to play the guitar instead of the keyboard and struggles with anxious, obsessive tendencies that she tries to soothe through music.

Madeleine is Oona's free-spirited, adventurous mother. A former flight attendant who loves to travel and date, she approaches life as an open experience and firmly believes in living in the present. She is one of the few people who knows about Oona's time-traveling condition and acts as an emotional anchor, though she deliberately withholds information about the past and future to force Oona to experience life naturally.

Dale is Oona's teenage boyfriend and the lead singer and guitarist of their band, Early Dawning. He is a handsome young man with a rockabilly style who prefers to take the lead in both their relationship and their music. He insists on playing the guitar himself, pushing Oona toward the keyboard, and offers her a leather jacket as a symbol of his protection.

Kenzie is a young, tech-savvy man who acts as Oona's personal assistant when she leaps into the 21st century. He helps her navigate modern technology like smartphones and iTunes, manages her daily life in the brownstone, and provides companionship. Born in the 1980s and raised by two mothers, he represents the modern era and helps translate the future for a woman who mentally belongs in 1982.

Edward is a charismatic chef whom Oona meets when she leaps into 2004. He introduces himself as her husband, explaining that they are in the process of opening a restaurant together called Clary's Pub. He accepts Oona's yearly memory loss as a medical condition and attempts to help her catch up on their shared life, though the stress of the failing restaurant frequently occupies his attention.

Peter is a Korean American guitar instructor who shares Oona's deep reverence for music. He wears a leather jacket and possesses a calm, steady demeanor that helps Oona finally pursue the instrument she has always wanted to play. He understands the sacrifices people make for their passions, having chosen music over a medical career.

Crosby is Oona's boyfriend when she unexpectedly arrives in the year 1991. He is a thoughtful, solicitous man who styles his hair in a pompadour reminiscent of Oona's first love. He arranges a formal birthday dinner for her, completely unaware that she just mentally arrived in this year and has no memory of their relationship up to that point.

Pam is Oona's best friend from elementary school. She represents the conventional, stable path forward in Oona's early life, having been accepted into the same study-abroad program in London that Oona is considering joining.

Corey is the drummer for Early Dawning. He struggles with substance use, a habit that creates friction within the band and prompts a strict reprimand from Oona right before her first time leap.

Wayne is a member of Early Dawning. He notices Oona's anxiety about her impending life decisions and offers a brief moment of concern during the chaotic celebration before her first leap.

Cyn is a transgender woman who becomes a core part of Oona's social circle during her 1991 leap. She introduces Oona to the intense New York club scene and patches her up after a dangerous physical altercation on the street.

Desi is part of the energetic New York nightlife crowd that Oona joins during her 1991 leap. He participates in the heavy drug use and impromptu dance parties that consume Oona's time that year.

Jenny is a drag queen who welcomes Oona into her social circle. She lives in a rougher part of New York and deals drugs in the Meatpacking District, supplying the substances that fuel Oona's year of escapism.

Francesca is a sophisticated restaurant consultant hired to help Edward open his business. She maintains a very close, familiar rapport with Edward and often acts as a gatekeeper to his office during stressful moments of the restaurant's development.