Jared is a nine-year-old boy struggling with his parents' recent divorce, which has led him to get into fights and neglect his schoolwork. Unlike his siblings who have defined hobbies, he feels aimless and often doubts his own intelligence. He is observant and curious, becoming fully absorbed in investigating the strange noises inside the walls of the Spiderwick mansion.

Simon is Jared's nine-year-old identical twin. He is orderly, respectable, and deeply invested in biology, maintaining a large collection of small animals like mice and tadpoles in glass tanks. Though generally well-behaved, he sympathizes with Jared's struggles and supports his brother when strange things begin happening in the house.

Thirteen-year-old Mallory is the oldest of the Grace siblings. She is a dedicated and champion fencer, a skill that gives her confidence and a quick temper. She takes charge of her younger brothers with a bossy attitude, initially dismissing the idea of supernatural creatures in favor of practical explanations.

Thimbletack is a tiny, pencil-sized brownie who lives inside the walls of the Spiderwick mansion. When his meticulously arranged cubbyhole is destroyed by the children, he transforms into a vengeful boggart. He communicates through rhyming notes and exacts petty, destructive revenge on the family members who offended him.

Helen is the recently divorced mother of Mallory, Simon, and Jared. She attempts to keep her family stable during a period of financial and emotional hardship by moving them into the dilapidated Spiderwick mansion. Unaware of the supernatural elements in the house, she struggles to manage Jared's behavior and interprets the magical vandalism as his acting out.

Arthur Spiderwick is the former owner of the Spiderwick mansion and the great-great-uncle of the Grace children. Decades ago, he used a hidden library in the house to study and document the fantastical creatures living on the property. His extensive notes and watercolor paintings form a comprehensive field guide that becomes essential to the children.

Lucinda is Arthur Spiderwick's daughter and the great-aunt of the Grace children. She currently resides in an institution because she insists that small creatures bring her delicious food. She owns the crumbling Victorian mansion and allows her niece Helen to move her family there during their financial struggles.