Which character is revealed to be Jacob’s twin brother in Lost?

The Man in Black is revealed to be Jacob’s twin brother in Lost. The character is introduced through earlier appearances and is later shown in the series mythology as Jacob’s brother from the island’s distant past.

After their mother dies, the brothers develop opposing relationships with the island and its mysteries. Jacob becomes associated with protecting the island, while his brother seeks to leave and eventually becomes the smoke monster. The show presents this transformation as a central part of its mythology.

The Man in Black frequently appears in the form of John Locke, but he is not actually Locke. Terry O’Quinn portrays both Locke and the Man in Black in those appearances. This identity is distinct from Richard Alpert and Benjamin Linus, two other major figures connected to Jacob.