Bett Devlin is one of the novel’s two protagonists. She is a 12-year-old girl from California who loves animals, surfing, and taking risks. Bett has a mind of her own, which often gets her into trouble, especially with authority figures. However, she is also highly intelligent and devoted to her family and friends. She can be impulsive and does not hesitate to argue when she thinks others are wrong.

Bett enjoys life as an only child of a single parent (Marlow) and does not initially want a new sister. At first, she focuses on the ways in which she differs from her counterpart, Avery. However, she comes to see the contrast between herself and Avery as a strength, not a weakness. Avery and Bett help each other through The Process of Growing Up and, in expanding their family, learn more about The Diversity of Family Structures and Found Family. They also both learn to strike a balance between exerting their own autonomy and respecting the autonomy of others (such as their fathers).