Grandpa is an elderly man experiencing progressive memory loss, which he visualizes as a shrinking physical space within his mind. A lifelong devotee of mathematics and logic, he uses games, maps, and equations to bond with his grandson. As his cognitive abilities deteriorate, he struggles to maintain his sense of identity while trying to prepare his family for his eventual decline.

Noah is an imaginative young boy who shares a unique, metaphorical language with his grandfather. He readily accepts his grandfather's shifting mental world without demanding logical explanations, treating it as a familiar game. Possessing emotional maturity beyond his years, he serves as an intermediary between his father and grandfather.

Grandma is Grandpa's late wife, appearing throughout the story as a vivid presence within his shrinking memory. While her husband gravitates toward abstraction and urgency, she focuses on everyday actions and shared routines. She acts as a stabilizing force in his mind, offering practical advice and reminding him of the life they built together.

Ted is Grandpa's adult son and Noah's father, bearing the difficult responsibility of managing his father's physical decline in the real world. He holds unresolved frustration toward his father for prioritizing work over family during his childhood. Despite this lingering resentment, he remains actively involved in his father's care and attempts to shelter his own son from the hardest realities of the illness.