Is Jack’s Dad more important to the mythology of LOST than we think? Consider these things. He is the first “vision of a dead person” that we come across on the island, right back at the beginning of the first season. Jack found his coffin, but there was no body in it, he didn’t find the body, assuming of course that Christian is actually still dead.
Christian has crossed paths with a lot of the survivors at different points, appearing in many flashbacks. He shared a drink with Sawyer, he hired Ana Lucia as a bodyguard and he is Claire’s Father (therefore Aaron’s Grandfather). They are just the one that come to mind instantly, there could very well be other that I’m forgetting, but I guess I’ll come across them in my re-watching.