Harold is an easygoing, food-motivated dog who serves as the story's narrator. He views himself as the dependable head of the household when his owners are away. Harold enjoys a comfortable routine and values the late-night snacks he receives from Toby.

Chester is an intellectual, well-read tabby cat with a highly active imagination. He prides himself on his reading abilities, especially his knowledge of horror and mystery novels. His exposure to vampire literature strongly influences his perception of the world around him.

Bunnicula is a tiny, timid rabbit with black fur markings that resemble a cape. He exhibits unusual habits for a rabbit, such as sleeping throughout the entire day and possessing sharp, pointy teeth instead of normal bunny teeth. He remains silent, leaving others to observe and guess his true nature.

Toby is the youngest son in the Monroe household. He is a playful, kindhearted boy who enjoys spending time with the family pets. He maintains a special bond with Harold by sharing his snacks in the evenings.

Peter is the older Monroe brother. He constantly seeks to assert his dominance in the family hierarchy by showing off to his parents and arguing with his younger brother. He is often self-absorbed and lacks the gentle nature his brother shows toward the animals.

Mr. Monroe is an English professor who brings an academic atmosphere to his home. He is a well-intentioned father who loves his family and pets, providing a comfortable life for them all. His vast book collection directly influences Chester's intellectual development.

Mrs. Monroe is the mother of the family and the primary peacemaker of the household. She acts quickly to resolve tensions, whether they involve her arguing sons or the strange behavior of the family pets. She keeps the home organized and ensures everyone's needs are met.