Winnie is an observant, creative fifth-grade student trying to survive her parents' highly competitive divorce. She feels ignored when they argue over keeping custody time completely equal, leading her to seek solace in a Wednesday routine alone in her treehouse. As her grades slip from endless obscure holiday celebrations engineered by her parents, she realizes she must find a way to make her voice heard.

Dr. Alexis Maraj is Winnie's mother and an academic mathematician. Locked in a fierce competition with her ex-husband, she insists that every aspect of Winnie's care is split exactly evenly. She goes to extreme lengths to outdo him, filling her custody days with bizarre, obscure holiday celebrations that leave Winnie exhausted.

Dr. Varun Malladi is Winnie's father, an academic researcher who studies bird feces. Like his ex-wife, he is extremely competitive and hyper-focused on keeping his custody time perfectly equal. His self-absorption prevents him from listening to Winnie's concerns, even declaring that she will spend her entire summer assisting him in the field.

Uncle Huck is an architect, Winnie's maternal uncle, and her father's best friend from college. He is one of the few supportive, nurturing adults in Winnie's life. He designs the treehouse to fit her tastes perfectly, complete with a zipline to his own home, and teaches her how to use Artist Vision to see the world more clearly.

Squizzy is one of Winnie's best friends and a key member of the Tulip Street Ten. She is studious, curious, and responsible. Within the friend group, she often acts as a confident, take-charge leader who playfully teases or corrects her peers.

Lyle is one of Winnie's best friends and a loyal member of the Tulip Street Ten. He is generally good-humored and goofy, yet highly attentive to Winnie's emotions. He provides vital encouragement to Winnie and notices when she is upset before others do.

Aayush is a classmate and friend of Winnie's in the Tulip Street Ten. While he initially voices frustrations about his parents' rules regarding a science project, his complaints mask a deeper desire for independence. He participates enthusiastically in the treehouse standoff.

Tabitha is a fifth-grade student and a member of the Tulip Street Ten. She joins Winnie in the treehouse to protest her own family rules, hoping to secure more respect from her parents by presenting them with a collective list of demands.

Logan is a member of the Tulip Street Ten, a tight-knit group of fifth-graders who have been in the same class since kindergarten. He moves into the treehouse to present his parents with a list of demands, eager to escape strict household rules.

Brogan is a fifth-grade student and a loyal member of the Tulip Street Ten. Along with his classmates, he occupies Winnie's treehouse to protest grievances against his family, experiencing both the initial excitement and the eventual fatigue of the standoff.

Greta is one of Winnie's classmates in the Tulip Street Ten. She participates in the treehouse siege, initially treating it as a fun slumber party before the stress of the protest causes tempers to flare among the group.

Joey is a student at Tulip Street Elementary and a member of the Tulip Street Ten. He joins his friends in the treehouse to demand changes from his parents, contributing specific requests to the list of requirements that the adults must meet.

Jolee is a member of the Tulip Street Ten who has shared classes with Winnie since kindergarten. She takes part in the treehouse strike, hoping to convince her parents to listen to her personal wants and needs.

Mr. Benetto is Winnie's fifth-grade teacher. He is observant of his students' fluctuating moods and academic struggles. He holds Winnie back after lunch to warn her that she is in danger of failing the year, offering her one final chance to pass by assigning a comprehensive local history report.

Buttons is Winnie's pet cat, whom she considers the absolute best in the world. He acts as an emotional surrogate for Winnie. When she feels angry or upset but cannot express it aloud to her parents, she projects her feelings onto him.