Jacob (Mark Pellegrino) was the mysterious authority figure who protected the island on "Lost." He and his fraternal twin, the Man in Black (Titus Welliver), were born on the island when their pregnant mother's ship crashed there. Their mother was murdered by a woman (Allison Janney) who protected the island and later took them to raise. She needed a replacement to follow in her footsteps as guardian of the island, a role that she passed to Jacob.
Jacob traveled off the island as he sought a candidate who would replace him as protector, and brought many people to the island over the years. Jacob died when Ben (Michael Emerson) killed him under instruction from The Man in Black who had taken over Locke's (Terry O'Quinn) body.
"It only ends once. Anything that happens before that is just progress."
"You were all like me. You were all alone. You were looking for something you couldn't find out there. I chose you because you needed this place as much as it needed you."