* Jack: A teenage boy who is the main protagonist and narrator of the story. He is a skilled swimmer and a passionate marine biologist, who becomes entangled in the terrifying events that unfold.
* Sarah: Jack's older sister and a skilled diver, who is initially skeptical of Jack's claims about a monstrous creature in the ocean.
* Dr. Roberts: A marine biologist who is researching the mysterious creatures in the ocean and becomes a key ally to Jack.
* The creature: A large, unidentified sea creature that is responsible for the attacks and terror in the story. While it's not a person, its actions drive the plot and shape the characters' destinies.
The story also features several supporting characters, including Jack and Sarah's parents, local fishermen, and other residents of the coastal town.