Blood Tide

A middle-aged treasure hunter who arrives with hostility toward Neil. He is willing to take dangerous risks, best demonstrated when he uses a homemade bomb to unseal a hidden tunnel. In the end, he sacrifices himself by detonating the cavern to destroy the serpent.

Neil's missing sister and an artist who traveled to the island. On the island, her personality begins to change, and she becomes trance-like as she uncovers disturbing paintings in the monastery. Her altered state drives much of the mystery and leads to her entry into the underwater cavern.

A determined American husband searching for his missing sister. He follows Frye into the underwater cave to uncover the truth, showing resourcefulness and a protective instinct toward Madeline. He helps pull Madeline from the cavern as Frye detonates the tunnel.

Neil's wife who accompanies him on the search. She notices Madeline's strange behavior and cares for those around her, including gifting a perfume to Madeline. She remains steadfast beside Neil as they confront the island's secrets.

Madeline's friend who joins them on their boat in a remote part of the island. She is attacked while swimming and killed, with her dismembered body later found by local children, underscoring the island's danger.

A young village girl who plays with the local boys and falls off a cliff into the water. Frye saves her from the sea, but her mother is later eaten by the serpent, highlighting the island's deadly mythology.

Lethe's mother who tries to save her daughter but is claimed by the serpent, illustrating the personal cost of the island's dangers.

The elderly mother superior of the convent. She appears in the island's religious setting and is found at the festival in a bloodied, shocked state after the convent is left in disarray with the nuns dead.

A nun at the convent who speaks with Madeline and witnesses troubling paintings uncovered in the walls. She embodies the island's religious atmosphere and mystery surrounding the paintings of saints and serpents.

The elderly mayor of the island who greets Neil and Sherry and represents the local authority and tradition. His presence anchors the community amid the unfolding investigation.