Ben is known for being sweet, socially awkward, and the older brother of Esther. He is a 13-year-old with shaggy brown hair, freckles, and a gap in his teeth, and he often deals with magical mishaps, including creating multiple versions of himself, like "Free Spirit Ben", "Horny Ben", "Confident Ben", "Cat Ben", and "Magic Ben". His personality is he tries to be cool but is often insecure and awkward, with a quirky relationship with his sister Esther.

Esther lives in the haunted hotel with her family. She is 11. And is a precocious and eccentric young girl who moved with her family into a haunted hotel, the Undervale, and uses black magic to navigate its supernatural challenges. She is the daughter of Katherine and sister of Ben, Esther is known for her fascination with the supernatural, her loyalty to her family, and her clashes with her mother and brother.

A recently divorced mother who inherits a haunted hotel from her deceased brother, Nathan, and must run it with her two children, Ben and Esther, while dealing with supernatural chaos. She serves as the grounded, emotional anchor for her family as they navigate ghosts and demons in their new home golden brown hair and brown eyes, often depicted wearing a casual brown jacket

Nathan who appears as a friendly but irresponsible ghost after dying and becoming the spectral owner of the Undervale Hotel, where he helps his sister Katherine and his nephew Ben navigate supernatural events. He is known for his optimistic nature, mustache, and close relationship with his nephew Ben, often teasing him while offering guidance.

Abbadon was Originally a powerful demonic entity, Abaddon was sent to Earth to cause chaos in his 10,000th year. When he possessed a young boy, he didn't realize the boy's father was a skilled priest. During a failed exorcism, the priest bound both Abaddon's body and spirit. They fell off a cliff during a struggle, and Abaddon has been doomed to wander the Earth eternally as an ageless, trapped nine-year-old boy. His personality is, Despite being a "High Prince of the Black Realm," Abaddon often acts as a dramatic, chaotic, yet endearing member of the hotel. He bonds with the daughter, Esther, over a mutual affinity for mischief, and frequently complains about being stuck in his mortal vessel. Has the mentality of a nine year old but still knows some things.