Reg is a robot who appears to be a little boy. His actual age, origins, and even name are unknown, as he can't recall anything about his life before his encounter with Riko in the First Layer of the Abyss. Riko named him Reg after a dog she used to have.

Reg's body is extremely durable and resistant to physical damage, and is much lighter than its size would suggest. His arms can be extended to be 40 meters in length, and each of them has a built-in Incinerator relic. These high-powered beam cannons are capable of reducing to ashes almost anything; however, his Incinerator is a weapon of last resort as even a single shot depletes the body's energy almost completely, which in turn makes Reg lose consciousness for 2 hours ten minutes after its use. Having an artificial body, Reg may be the one of the only intelligent beings completely immune to the Curse of the Abyss. Because of this, he is sometimes referred to as an "Aubade" or the "Treasure of the Deep".

Despite being a formidable fighter, Reg is rather shy and unassertive. He blushes easily and often cries when in distress. Unlike Riko, Reg becomes easily flustered when in danger, and must often rely on Riko's judgment in difficult situations.

(Source: Wikipedia, edited)