Harrison Sadler arrived in New England from England studying ghosts and the occult. He found just what he was looking for when he discovered the spirits of three children- Sam, Louisa and Nat Sadler, who all died in the winter of 1641. He spent the rest of his life in a nearby cave, hoping to one day seal the ghosts inside forever.
When siblings Jerry and Terri Sadler first encounter him, they believe him to be a ghost. During their second encounter, he reveals the truth to them about their three new friends, but they are skeptical. Terri eventually leads the trio of ghosts to him, and he sacrifices himself by sending the rocks on top of the cave crashing down in front of the entrance, trapping the ghosts within.
Harrison seems very curious about the supernatural, and that has led to him to an obsession of wanting to trap the trio of ghosts. He seems to care about the safety of Jerry and Terri, and makes a selfless sacrifice when he finally traps Sam, Nat, and Louisa.
Harrison is described as having stringy white hair, and a beaklike nose. He is very thin, and pale, and his eyes are very dark.
- In the Portuguese adaptation of this book, Harrison Sadler's name changes to 'Henrique Diniz', in order to adapt to the language.