Evan is a socially adept boy preparing to enter the fourth grade. He lacks confidence in math and feels insecure about his academic abilities, especially compared to his younger sister. Though he typically acts as a caring older sibling, his frustration over sharing a classroom drives him to declare a high-stakes lemonade selling competition.

Jessie is a highly academic student who is skipping third grade to join her brother's fourth-grade class. She excels at math and puzzle-solving but has significant difficulty with social cues and understanding the emotions of others. Following a rough second-grade year with mean classmates, she uses the lemonade competition to test out business theories and attempt to build new friendships.

Mrs. Treski is a newly single mother working as a public relations consultant from her home office. She balances the demands of her career with raising two children, providing them with business pamphlets that inspire their sales tactics. She maintains a hands-off parenting approach to their arguments, encouraging them to develop empathy and solve their own conflicts.

Megan is a fourth-grade student in Evan's class who agrees to start a lemonade stand with Jessie. She proves to be a supportive and generous partner, introducing positive social strategies like writing comment cards to boost confidence. She expresses a desire to donate her lemonade profits to the Animal Rescue League.

Scott is a boy in Evan's fourth-grade class who frequently behaves selfishly. He calls Jessie "freaky" for skipping a grade and drinks most of the inventory while partnering with Evan on their first lemonade stand. His presence creates tension for both Treski siblings.

Mr. Treski is Evan and Jessie's father who recently left the family. While not physically present during the summer, his departure drastically shifted the household dynamics. His absence causes his children to actively hide their conflicts to spare their mother further grief.

Carly is a girl from Evan's fourth-grade class who spends time with Megan. She gives Megan unexplained looks during conversations, triggering Jessie's anxiety about mean girls from her past.

Paul is one of Evan's reliable friends. He helps Evan execute a highly successful, low-cost lemonade stand by selling cups for ten cents, relying on customer goodwill to generate larger tips.

Officer Ken is a local police officer who patrols the town square. He enforces local ordinances by informing Evan that selling goods in the commercial district requires a permit. He softens this rejection by purchasing a five-dollar cup of lemonade himself.

Jack is a neighborhood boy who plays basketball with Evan and the others. He invites the group back to his house to swim in his pool to escape the late summer heat.