Reg is a robotic boy discovered unconscious in the Abyss's first layer by Riko, who named him after her deceased dog. His origins are obscured by amnesia, though fragmented memories suggest he ascended from the depths. A sketch by Riko's mother, Lyza, indicates she encountered a similar entity on the seventh layer. He possesses light brown skin, dark hair, yellow slit-pupiled eyes, pointed ears, and distinctive red rectangular patches on his cheeks and rib cage. His body blends human-like features—including nipples, a belly button, and genitals—with mechanical lower arms, legs, hands, and feet. His attire consists of a tattered red cape, baggy green-and-purple pants, and a metal helmet with asymmetrical horns and a lens displaying charges for his primary weapon, the Incinerator.

Timid and naive, Reg exhibits fierce protectiveness towards Riko, blushing and crying easily but displaying hot-headed determination in crises. His speech patterns and mannerisms, including frequent use of "irredeemable," echo Lyza's traits. Abilities include immunity to the Abyss's curse, extendable arms for grappling up to 40 meters, heightened senses for scent tracking and visual zoom, and concussive force generation from his palms. His Incinerator—a beam weapon in each limb—vaporizes targets but depletes energy reserves, causing two hours of unconsciousness per use. He recharges via electricity or food, though his cooking is inedible. A thinner armor section at his navel presents a physiological weakness.

Fragmented pre-amnesia memories reveal he befriended Faputa, the "Princess of Hollows," in the sixth layer, gifting her his helmet goggles and promising to return. He also aided the damaged Interference Unit Gaburoon. Revived by Riko in Orth, he joined her descent. Ozen the Immovable provoked him to test his combat readiness, exposing his overreliance on the Incinerator and poor tactical coordination with Riko. In the fourth layer, Reg attempted to amputate Riko's poisoned arm, leaving a prominent scar.

During the fifth layer events at Ido Front, Bondrewd captured Reg for experimentation, severing his right forearm. This injury later accommodated the artifact Gentle Knock. Forced into a berserk state after absorbing Ido Front's energy reserves, Reg fought Bondrewd with feral intensity; his animation mirrored the blurred, unnatural style of Abyss monsters, contrasting Bondrewd's precise depiction, underscoring his loss of control. Post-battle, Reg acquired a scar across his navel.

Descending deeper, sporadic memories resurface—holding Lyza's Blue Pendant, encountering eternal fortunes, or sharing meals—hinting at a complex past intertwined with the Abyss's deepest secrets.