Main Characters:
* Charlie "Chuck" Lewis: The protagonist, a lonely, eleven-year-old boy who feels like he's not getting enough attention from his family. He's obsessed with birthdays and is on a mission to figure out the "birthday code."
* Kate "Katie" Lewis: Chuck's older sister, who is turning thirteen and feels like her life is falling apart. She's dealing with a complicated relationship with her boyfriend, her parents' growing distance, and the general stresses of adolescence.
* Sam Lewis: Chuck and Kate's younger brother, who is eight years old and has a lot of energy.
* Grace Lewis: Chuck, Kate, and Sam's mother. She's busy with her job as a doctor and struggles to balance her career with her family life.
* David Lewis: Chuck, Kate, and Sam's father. He's a writer and often seems distant and preoccupied with his work.
Supporting Characters:
* Mr. and Mrs. Tremont: Chuck's eccentric neighbors, who have a unique perspective on birthdays and life.
* Sarah: Kate's best friend, who is also turning thirteen and is struggling with her own relationship issues.
* Mr. and Mrs. Bell: Chuck's school principal and his wife, who have an interesting story of their own about birthdays.
* Mrs. O'Malley: The school librarian who is helpful and encouraging to Chuck.
This is a basic overview, and there are other, smaller characters that play a role in the story. However, these are the key players and their relationships are central to the themes explored in the novel.