Grace is an analytical, scientifically-minded sixth grader living in Illinois. She loves formulating theories about the world and collecting unique objects. A bit shy and unassuming, she often lets her outgoing best friend, Ellie, take the lead in social situations, though a new school fad prompts her to reevaluate their friendship.

Ellie is Grace's long-time best friend, known for her magnetic, outgoing personality. She has a domineering streak and frequently uses her charm and social status to place herself at the center of attention. She enjoys being a trendsetter but struggles with jealousy when others receive the spotlight.

Hank is a smart, self-assured sixth grader who prefers conducting his own research to following the crowd. He offers a steady and supportive presence in Grace's life. Unlike Ellie, he does not seek approval from others and is content making honest trades and exploring his own interests.

Grace's grandfather is a widower dealing with the grief of losing his wife a year ago. To keep himself busy, he purchases a dilapidated mill building in Boston to refurbish. He shares a close bond with Grace, sending her boxes of antique buttons that unexpectedly spark a major trend at her school.

Grace's brother is a supportive sibling who engages with her analytical mindset. He helps her think through the economics and societal implications of the school's new button fad, introducing concepts like supply and demand to explain human behavior.

Grace's mother provides a mature perspective on the shifting nature of relationships. By sharing her own experiences of losing touch with childhood friends, she helps Grace understand that growing apart is a natural part of growing up.

Brooke is a classmate of Grace and Ellie whose possession of a highly unique pinwheel button temporarily makes her the most powerful person in their social circle. Her interactions highlight the shifting power dynamics caused by the school's new fad.