A 12-year-old girl. Her Delver rank is Red Whistle. She is characterized by her blonde pigtails, exposed forehead, and glasses. She is the daughter of the legendary White Whistle Lyza the Annihilator and Torka, a member of Lyza's delve team. To avoid the risk of kidnapping that comes with being a hero's daughter, she was separated from her parents at birth and raised as an orphan at Belchero Orphanage in the town of Orth, located at the edge of the Abyss. She finds the Star Compass during her journey into the Abyss.

A boy-type robot found by Riko in a deactivated state within the Abyss. He has lost his memories and doesn't even remember his own name or the fact that he is a robot. Because he is a robot, he is completely immune to the Curse of the Abyss (the Strains of Ascension). He aims for the bottom of the Abyss with Riko to discover his true identity.

Nnaaa...!? You guys... stop staring... it's embarrassing... Hey AI, Nanachi isn't Faputa, okay? I don't have four tails. I have two arms, right? And I live on the fourth layer.

One of the living legends known as White Whistles, with the title "Lord of Dawn" or "The Novel Bondrewd." He operates out of Ido Front, a forward base located in the fifth layer, the Sea of Corpses. He wears a mask and a power suit, appearing like a robot and never showing his face or skin. His delve team, the Umbra Hands, also wear masks and suits, but their mask designs vary, making them easy to distinguish. He has a daughter named Prushka. He often says things like "Oh my, how very interesting."

...No... Prushka is spilling out...

An incredibly large and violent woman. Ozen's primary first-person pronoun is "Watashi" (polite/formal). She has a disciple named Marulk. Marulk uses "Boku" (soft/masculine) and has an androgynous appearance with blue hair. Marulk is a child who serves as a lookout at the Seeker Camp in the second layer of the Abyss and holds the rank of Blue Whistle (an adept delver permitted to enter the second layer). Marulk usually wears a maid-like outfit. Marulk lives with Ozen, the master of the facility and its guardian. Marulk has been at the Seeker Camp since being rescued by Ozen years ago, primarily because Marulk has a physical constitution that is weak to sunlight and cannot live on the surface. While Marulk works as a live-in disciple, it is said that Ozen keeps Marulk there out of concern for the child's well-being. Marulk calls Ozen "Master" and looks up to her. Marulk is originally from Jesku, a country far to the west of Orth, and was the sole survivor of an expedition that attempted the Abyss by airship eight years ago. Marulk is likely around the same age as Riko (12). Although regulations state one cannot become a Blue Whistle under age 15, Marulk was granted the rank as an exception for being a direct disciple of a White Whistle. Marulk is serious and polite but still immature as a delver, with a fragile psyche similar to or even more so than Reg, having retired halfway through the survival training that Riko and Reg completed. However, Ozen praises Marulk's judgment, saying, "As expected of my disciple." Due to the sunlight sensitivity, Marulk uses a parasol when going to Orth on errands. Marulk's tone is soft, often saying things like "Master rescued me when I collapsed after an accident" and "I've been here ever since. About 40% of what we find here are egg-shaped artifacts. It's strange, even though they're tools used by people long ago, isn't it? I know! I'll try asking Master if you can stay and delve here a bit longer."

A mysterious creature living near Ilblu, the Village of the Hollows, in the sixth layer of the Abyss, the Capital of the Unreturned. She has brown skin, golden eyes, and hair and ears that flare outward like wings. She has ruby-like horns where a tiara would sit. While her facial features and overall body shape are close to a human child, she has four arms with sharp claws. Her arms and lower body are covered in white feathers, and she has five fluffy tails like a nine-tailed fox, which look like the Eternal Fortune flowers from behind. Her three sharp claws on each arm are dexterous enough for precision work, and she hides a fourth claw where sensory organs are concentrated.