An old cemetery caretaker who knows too much about the Ghost Rider, for the simple reason that he was one.
Carter Slade is the Caretaker, which is how the credits of Ghost Rider list him. Sam Elliott plays him, and narrates the film. He is a former Texas Ranger from the nineteenth century and the first Ghost Rider, the one who was sent for the contract of San Venganza, saw what it would give the Devil, and hid it instead. To Johnny he is an ally and a mentor: the man who tells him that what happened to him was real, that it will happen again at night near an evil soul, and that a predecessor named Carter Slade hid the contract inside a spade.
Minor spoiler
When Johnny comes for the contract to save Roxanne, the Caretaker gives it to him, tells him he is stronger than the Riders before him because he sold his soul for love rather than greed, and reveals that he is Carter Slade. He rides with Johnny to San Venganza, hands him a lever-action shotgun, and having finally shaken off the curse, fades into dust as he rides away.