A 12-year-old girl. Her Delver rank is Red Whistle (apprentice). She is characterized by her blonde pigtails, exposed forehead, and glasses. While she wears glasses, it is due to a physical condition where she suffers headaches unless she views the world through crystal lenses; her actual eyesight is not poor. Her mother is Lyza, a "White Whistle" hero of this world. She possesses extensive knowledge of the Abyss's flora, fauna, and relics. Regarding relics specifically, she has memorized the entire "Relic Catalog," a list of all relics ever made public. Beyond just memory, she excels in application and observation, allowing her to navigate unforeseen situations through improvisation.

A boy-type robot found deactivated inside the Abyss by Riko, an apprentice Delver. Being a robot, he is completely immune to the "Curse of the Abyss" (Ascension Strain). He is an exquisitely crafted robot whose face and torso are nearly indistinguishable from a human's. He is a boy with short brown hair and golden eyes, with skin slightly darker than Riko's. His ears are pointed, and he has reddish-brown square patterns on his cheeks and sides. His most striking features are his limbs, made of lustrous metal. His elbows and fingers are silver bellows, while his forearms and shins are sturdy, iron-black cylinders. Both arms can be fired at high speeds, extending nearly 40 meters on wires about the thickness of a fist. "Incinerator" He fires a destructive beam from lens-like parts in his palms and the soles of his feet. It is also called a "heat ray" because it leaves scorched marks where it hits.

A White Whistle Delver currently serving as the guardian of the "Seeker Camp" in the second layer of the Abyss. Her titles as a White Whistle are "The Immovable" and "Ozen the Unmovable." She lives with her beloved disciple Marulk and serves as the leader of the Delver squad "The Subversives." Standing over two meters tall and dressed in black, she resembles a giant shadow. However, she often maintains an extreme slouch when seated, making her height less apparent. When speaking to others, she hunches so severely that her back rises above her head to meet their eye level. Her rounded back stays high while her head sits slightly lower, all draped in black clothing. Though she appears to be in her 30s, over 50 years have passed since she earned the title of "White Whistle." Combined with Marulk's statement that one can become a Blue Whistle (a full-fledged Delver) at 15, her actual age is at least 65, barring exceptions like skipping ranks. She enjoys alcohol and has a taste for delicacies and strange foods. Personality-wise, she is known as a difficult individual, even among other White Whistles. She hurls extraordinary insults in a flat, drawn-out tone and openly states she "hates childish tricks," showing no kindness even to young children. When delving, she wears a cloak that covers almost her entire body, along with giant gauntlets and a wide, circular helmet resembling a war hat. She utilizes the First-Grade Relic "Thousand-Men Wedges." The effect is "gaining the strength of a thousand men with a single stake." Ozen has driven 120 of these into her own body.

One of the living legends among the "White Whistles," known by the titles "Lord of Dawn" and "Bondrewd the Novel." He usually operates out of Ido Front, a forward base located in the fifth layer, the "Sea of Corpses." He appears like a robot wearing a mask and a powered suit, a mysterious figure who never shows his face or even his skin. His subordinate Delver squad, the "Umbra Hands," all wear masks and powered suits as well, though their mask designs vary greatly, making them easy to distinguish. He also has a daughter named Prushka. He is a scientist dedicated to research at Ido Front to unravel all the mysteries of the Abyss. He frequently uses the catchphrases "Oh my, oh my" and "Marvelous." He appears on the surface to be a soft-spoken philanthropist. In reality, he is a mad scientist akin to an alien, whose nature is incompatible with common human values. While he possesses a gentlemanly demeanor, he is a psychopath lacking the capacity for consideration or empathy. Defining both his brilliance and his abnormality is his invention, the "Cartridge." This is a revolutionary item that negates the effects of Ascension Strain when equipped; however, its true nature is a human child stripped of everything but the essential organs and packed alive into a box. Troublingly, Bondrewd believes these atrocities are the foundation for humanity's progress, acting without a shred of malice or ill intent.

A demi-human with a youthful appearance. They live in a house built on the edge of the fourth layer where the strain is weak, residing with a "Hollow" named Mitty. Possessing knowledge gained while trying to save their friend Mitty and wisdom from traversing the fourth layer, they have a realistic, mature, and calm personality. However, they also possess the humanity to help Riko and her friends despite the risk of their secrets being exposed. This personality stems from being an orphan with a traumatic past. They have a dull sense of taste and are a poor cook as a result. Because they have the appearance of a demi-human with rabbit-like ears and are young, and because the work has not explicitly stated it, their gender is currently unknown. Their actual age may be somewhat higher than their appearance suggests, but it is not precisely known. They refer to themselves as "Oira" (casual/masculine) and use masculine speech patterns like "-daze," and their steady courage often leads others to perceive them as a boy.