The Otherworld is considered to be a unusual place where battle never ceases. Among the inhabitants of the Otherworld are two beings: one who wishes to destroy the balance, and the one who strives to protect it.
However, on this side of the Otherworld, things are different.
Black★Rock-chan focuses on an unusual relationship between Rock-chan, the protector of the balance of the Otherworld, and De-chan, her presumed nemesis-turned-babysitter.
- Rock-chan (ろっくちゃん Rokku-chan) - a super-deformed version of Black★Rock Shooter and the protector of the balance in the Otherworld. Compared to her non-caricature version, Rock-chan is clumsy, childish and always hungry.
- De-chan (でちゃん De-chan) - she is considered to be the disruptor of the balance of the Otherworld and is the primary enemy of Black★Rock Shooter. However, she turns out to be the one who would always cook for and play with Rock-chan. Despite this, the admits that she cannot control her strength, and often frightens Rock-chan, with comedic results. Later in the series, it is mentioned that she is actually Rock-chan's mother.
- Doku and Kuro - two silent skull-shaped floating creatures which follow De-chan. Despite belonging to De-chan, Doku and Kuro often take Rock-chan's side whenever Rock-chan and De-chan play to help look after her. The two also act as Rock-chan's guardians when De-chan is away.
- Strength (nickname Su-san ) - Rock-chan's friend. She possesses a tendency to do things her way, and often tests Rock-chan's patience. She often uses her robotic arms to toss objects (even Rock-chan herself) into the air, and is even more destructive when throwing a tantrum. Her personality causes Rock-chan to take up an older sister role.