Willow is a 12-year-old highly gifted girl who approaches the world with a unique analytical perspective. Adopted by Roberta and Jimmy Chance, she often feels she does not quite fit in with her peers at school. She is fascinated by medical conditions and the number seven, while maintaining a deep passion for botany. Following a devastating family tragedy, she must find new ways to connect with the people around her and rebuild her sense of belonging.

Dell is a chubby and bearded school counselor who lacks motivation for basic adult responsibilities. He is prone to hoarding junk in his apartment and uses a rudimentary color-coded system to label his students. His apathetic daily routine is upended when he meets Willow, a student whose brilliant mind forces him to actually pay attention to his job and rethink his isolated existence.

Mai is an assertive 14-year-old high school freshman. Despite being the younger sibling, she acts as a fiercely protective keeper for her older brother. Quick-witted and highly persuasive, she is not easily intimidated by adults and quickly steps into the role of Willow's protector when the younger girl faces a massive crisis.

Born Dung in Vietnam, Pattie is the hardworking and pragmatic owner of a nail salon in Bakersfield. She lives frugally with her children in a garage behind her business. Highly organized and fiercely dedicated to her family, she relies on practical solutions while harboring superstitions about luck. Despite her strict exterior, she possesses a deep well of empathy for outsiders.

Jairo is a taxi driver in his thirties who has been driving for Mexicano Taxi for seven years. Comfortable but uninspired by his daily routine, he experiences a wake-up call when Willow becomes his regular passenger. Her intuitive medical observations and life advice make him reconsider his choices and think about returning to college.

Quang-ha is a high school sophomore labeled as a troublemaker by school administrators. Sullen and emotionally guarded, he often resents his family's cramped living situation and feels that life is unfair. Though he puts up a disinterested front, he is highly observant and possesses a hidden talent for art.

Roberta is Willow's adoptive mother. After struggling to conceive for seven years, she and her husband adopted Willow to create a small and close-knit family. She loves Willow deeply and tries to gently guide her unusual daughter toward more typical social behaviors, going to great lengths to shield her from worry.

Jimmy is Willow's adoptive father. Together with Roberta, he provides a loving and unconditionally supportive home for their highly gifted daughter. He ensures Willow feels safe and accepted despite her eccentricities.

Lenore is an overwhelmed but well-meaning caseworker for the Jamison Children's Center. She is tasked with managing Willow's case and working through the complex foster care system to find a suitable permanent placement for the grieving child.

Sadhu is a resident at the Gardens of Glenwood who occasionally interacts with Dell. He becomes involved in the characters' elaborate logistical shuffling when Dell needs to quickly find a new living arrangement to maintain a facade.

Henry is the owner of a local plant nursery. He shares Willow's appreciation for botany, eventually becoming a quiet supporter of her ambitious courtyard gardening project by providing necessary supplies.

Cheddar is an orange cat that Dell brings to his office in a misguided attempt to connect with his students. His sudden escape bonds Willow, Mai, and Quang-ha as they search the grounds, acting as a recurring catalyst for responsibility.