If I were to agree with the idea that every Lost character must have some constant for traveling from & to the island, I'd say it can well be Richard Alpert for Ben. On the island, Richard seems to be the face he knows for longest and he is the one who made Ben's 'rebirth' possible (I'm referring to the mass poisoning of Dharma people carried out by Richard & Ben). We've also seen Richard off the island on 3rd season as well (Juliette episodes) so Richard might work for Ben if he were ever to need a constant.

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"Bear...? Is that you?"-- Hurley S03E4