Muzan Kibutsuji

Age: <20 (Human), ≈1,000 (Chronologically), >10,000 (Biologically)
Height: 179 cm (5'10½")
Weight: 75 kg (165 lbs)
Affiliation: Twelve Kizuki (Leader)

Kibutsuji Muzan is a Demon, the first of his kind, and the progenitor of most other Demons.

Muzan was turned into a demon by a plant called the blue spider lily, and cannot die by being beheaded. His goal is to overcome his only weakness, which is sunlight.
(Source: Kimetsu no Yaiba Wiki)