1 Nephi 11:12

12 And it came to pass that he said unto me: Look! And I looked as if to look upon him, and I saw him not; for he had gone from before my presence.

I used to play with the idea that this verse, coupled with verse 14, where an angel came to replace the Spirit of the Lord, was evidence that the first visitor was the pre-mortal Lord Himself, that His spirit could only be in one place at a time, and it was needed to show the rest of the vision about Him. Therefore, a replacement was needed to talk to Nephi. The Lord’s spirit was needed to be in the vision about Him, and since He couldn’t be in the vision and explain it at the same time, an angel came to do the explaining. Now, I suppose this might be the case, but I have never heard or read anything else about that. So I don’t necessarily subscribe to the idea, but Nephi seems to make it very clear that he was visited by two separate Beings. If the first visitor was the Holy Ghost, he wouldn’t need to leave, since his job is to testify anyway. I had this question while serving my mission. James E. Talmage and Bruce R. McConkie didn’t agree on this, one saying it was Christ, the other saying it was the Holy Ghost. The footnotes used to cross-reference Nephi’s Spirit of the Lord with the brother of Jared’s Spirit of the Lord, which was definitely Christ. The version of The Book of Mormon we currently have doesn’t make that reference. So we’re left to our own now, but my best guess is that this Spirit of the Lord is Christ.