What is the name of the primary antagonist who appears as the Smoke Monster?

The primary antagonist who appears as the Smoke Monster is the Man in Black. For much of Lost, the black cloud is treated as a mysterious island creature, but the sixth season reveals that it is the supernatural form of Jacob’s brother.

The Man in Black was originally human and lived on the island in ancient times. He became Jacob’s ideological opposite: he wanted to leave the island, while Jacob believed people could prove themselves capable of overcoming corruption. After Jacob threw him into the island’s mysterious source of energy, his consciousness emerged as the black smoke entity.

The Man in Black can scan people’s memories, judge them, and imitate the appearance of the dead. He appears as figures including Christian Shephard, Yemi, Alex Rousseau, and John Locke. His Locke form allows him to manipulate the survivors while concealing his true identity.

His ultimate goal is escape, but Jacob and later island protectors prevent him from leaving. The finale makes him mortal, enabling Kate and Jack to defeat him before he can destroy the island and reach the outside world.