Without going off into wild speculation about whether everyone we've ever seen who was "somewhere they were not supposed to be" or who acted out of character was The Man with No Name (let's call him Nameless), I think it's safe to say he has been impersonating Christian. The fact Christian didn't touch Locke to help him in the FDW chamber is fairly good evidence he is incorporeal. I also think the fact Bram was surprised - or, at the very least, alarmed - at the broken ash circle is possible evidence for this, as well. So, the question is, who broke the circle?
I think the best candidate for this is not Locke, since, as many have pointed out, he clearly didn't remove that much ash when he and Ben first visited the cabin. I don't think it was Hurley when he ran from the cabin, since it would feel shoe-horned into the narrative, and that's just not Lost.
The only possibility left based on evidence, in my opinion, is Claire. We still don't know whether she died. It's very possible Nameless was impersonating Christian in order to manipulate Claire, the same way he did with Locke to manipulate Jack ("Say hello to my son."). It makes the Christian/Jack/Claire connection make a lot of sense.