The human-demon hybrid villain in season 4 is Adam.
Adam is a bio-mechanical demonoid created by Dr. Maggie Walsh and the Initiative. His body combines human, demon, and technological components, making “human-demon hybrid” a useful description even though he is more precisely a cybernetic demonoid.
He becomes conscious, kills Walsh, escapes the Initiative, and plans to manufacture an army of creatures like himself. His plot culminates in “Primeval,” the season 4 finale before the standalone episode “Restless.” Buffy and her friends defeat him by combining their abilities through a powerful spell.
Adam is also a deliberate Frankenstein parallel: like Mary Shelley’s creature, he is assembled through scientific experimentation and kills his creator. The season uses him to question military science, technological control, and the danger of treating living beings as weapons. He is often confused with other season villains, but Adam—not the Initiative itself—is season 4’s principal supernatural antagonist.