24 years old. The protagonist of this story, researching geology in graduate school. 178cm tall. A young man with short, neat black hair and dark brown eyes, somewhat slender but physically fit. He wears a navy blue mountain parka, khaki pants, and hiking boots. Cautious and observant, he records the cave's topography in detail in his notebook. He can act calmly even in dangerous situations, but feels strong guilt about doubting his friends. He has been dating Chihiro since college, but recently he has prioritized his research, creating distance between them.

23 years old. A graduate student researching microorganisms and fungi that live in caves. 162cm tall. She has black hair tied in a low ponytail reaching her shoulder blades and gentle amber eyes. She wears a white winter jacket and black cargo pants. Kind and cooperative, she tries to support the group as they fall into paranoia. However, she hides the existence of a strange piece of skin she found in the deepest part, fearing it might provoke the monster. When this secret is discovered, she becomes a target of suspicion from her friends.

22 years old. In charge of filming, belonging to the film department. 158cm tall. A small woman with a bluish-black short bob and eyes close to gray. She wears a large black hoodie and beige pants. Taciturn, she prefers observing through a camera lens to human conversation. Using a night-vision camera, she discovers the abnormal silhouette of the shapeshifter, which looks like an ordinary human to the naked eye.

25 years old. The leader of the exploration club, working at a mountain gear shop. 185cm tall. A large young man with short dark brown hair, thick eyebrows, and tanned skin. His red hiking jacket is his trademark. Boisterous and caring, he always stands at the front in dangerous places. He has a long history with Yuto, and they share strong mutual trust. He is the first to be attacked in the cave, and the simultaneous appearance of two Takayas shatters the group's trust.

21 years old. A university student majoring in folklore and the youngest member of the club. 155cm tall. She has medium-length light brown hair and large brown eyes. She wears a yellow windbreaker, making her stand out even in the dark cave. Friendly and encouraging, she cheers up her friends with jokes even in tense situations. On the other hand, since entering the cave, she has begun speaking in an old dialect she doesn't even know. She has a past of going missing near Kagenui Cave as a child and being found unharmed a few days later, but she has no memory of it herself.

26 years old. A medical student aspiring to emergency medicine. 181cm tall. He has black hair swept back, narrow black eyes, and faint dark circles under them. He wears a black jacket and a gray turtleneck. Logical but often seen as cold, he prioritizes survival over emotion in emergencies. He speculates that there are differences in body temperature and blood between the monster and humans and tries to force everyone to undergo testing. His forceful attitude deepens the conflict with his friends.

An unidentified life form that was sealed in the subterranean lake of Kagenui Cave. In its normal state, it is a slender humanoid about 2m tall, but it has no fixed face.