Henry is a twelve-year-old boy who relocates from Boston to Kansas after his parents are kidnapped during a bicycle tour in South America. Bright but shy, he feels out of place in the lively Willis household and yearns for connection. Shortly after moving into his attic bedroom, he discovers dozens of mysterious, locked cupboard doors hidden behind the plaster of his wall.

Henrietta is the middle Willis child, an eleven-year-old girl with thick brown curls and green eyes. She strongly dislikes being called "Henry" to avoid confusion with her cousin. Unlike Henry, she possesses a bold, defiant curiosity and takes an immediate, eager interest in uncovering the secrets of the hidden attic doors.

Frank is Henry's thin, oddball uncle who operates an unusual online business selling tumbleweeds. He maintains a warm, welcoming demeanor toward his nephew and actively attempts to bond with him. He harbors a deep, unresolved fascination with his deceased father-in-law's stubbornly locked bedroom.

Dotty is Henry's aunt, a vivacious and loving woman who provides stability in the chaotic Willis household. She consistently works on domestic tasks, such as pulping apples, and ensures Henry feels comfortable in his new environment. She serves as a grounding presence for her eccentric husband and energetic daughters.

Nimiane is a dangerous, blind witch imprisoned in a dark world accessed through a black cupboard. Though physically deteriorating with sores and stubble for hair, she possesses the ability to cast illusions, making herself appear as a beautiful woman to manipulate others.

Anastasia is the youngest Willis child, a small, wiry nine-year-old with reddish-brown hair and freckles. She is a persistent meddler who sneaks around the house listening at doors to uncover her siblings' secrets.

Penelope, known as Penny, is the oldest Willis daughter. With black hair and a more mature demeanor, she often tries to manage her younger sisters' behavior. She approaches strange situations with caution and maintains an interest in a local baseball player.

Simon is the late grandfather of the Willis girls, who passed away two years before Henry's arrival. He left behind a mysteriously locked room and hidden journals detailing his investigations into the strange cupboard network built into the house.

Eli is a short, elderly, bald man who secretly resides in Grandfather's locked room. He is familiar with the cupboards and possesses knowledge of their magical properties, occasionally erasing the memories of those who spot him sneaking around the house.

Richard is a skinny, pale boy living a restricted life under the care of servants in another world. He encounters Henry through one of the cupboard passages and eagerly joins the investigation, desperate for friendship and a break from his sheltered existence.

Zeke is the best young baseball player in the town of Henry, Kansas. He willingly lends Henry a glove and helps him practice, eventually becoming entangled in the Willis family's supernatural conflicts when he visits their house for batting practice.

Blake is the Willis family's house cat. He demonstrates an unusual awareness of the magical energies in the home, often reacting aggressively toward dangerous supernatural intrusions and attempting to protect the children.

Phil is a travel writer currently on a bicycle tour across South America. His sudden disappearance following a kidnapping forces his son to relocate to Kansas.

Ursula is a travel writer and Dotty's sister who was kidnapped for ransom in Colombia, prompting Henry's move to the Willis household.

Darius is the leader of the Witch-Dogs and the "First among the Lastborn Magi." He corresponds through the magical mailbox, issuing cryptic demands and threats regarding the newly reopened cupboard doors.