A 12-year-old girl. She is highly inquisitive, with her desire to know often outweighing her fear. Triggered by a letter from her mother, she is a Red Whistle who sets out on a journey to explore the Abyss alongside Reg, a robot boy who has lost his memory.

While he has an appearance close to a human boy, his arms and legs are mechanical. He is not a simple robot; he eats, sleeps, feels pain, and reacts to poison and electricity, possessing senses. Appearance: Light brown skin, dark brown hair, slightly pointed ears, yellow cat-like eyes, and thin gray eyebrows. He usually doesn't wear a shirt, instead donning a worn-out red cloak. Discovered in the Abyss, he is a companion to Riko on her exploration and has a strong protective instinct toward others.

A boy who is an apprentice Red Whistle Cave Raider from the orphanage in Orth where Riko and the others live. He is a boy around Riko's age and shares a relationship similar to childhood friends. Rather than being at the center of the Abyss exploration and adventure, he is a boy who belongs to the daily life of Orth.

A member of the Belchero Orphanage and a peer of Riko. He is an apprentice Red Whistle Cave Raider. He exists as Riko's friend, providing knowledge about the Abyss and supporting Riko and Reg's departure rather than stopping them. While he appears to be a model student at first glance, he also has a mischievous side.

Considerably smaller than 12-year-old Riko and her friends Nat and Shiggy, his speech is still faltering due to his young age. He is a boy living at the Belchero Orphanage. Despite his youth, he is clever and well-behaved. His name is "Kiyui," though it is sometimes mistaken for "Kiwi." He is like a disciple to Riko.