He is the Demon King, the first of his kind, and the progenitor of all other demons in existence.

Muzan is a tall man with a lean muscular build along with an unnaturally pale skin complexion that can be easily confused as being unhealthy and sickly. He has curly black hair and sharp, almond-shaped eyes. His irises are cat-like in appearance and are of a plum red color. He also possesses sharp, blue nails on his hands. Physically, he appears to be in his twenties.