Who is "Samuel" in the story of Saul, Samuel, and the Medium?

Mike gives two possible interpretations here. There's a third one as well: that "Samuel" here is... nobody at all.

Keep in mind that the witch is the only one who "saw" Samuel, and she had to describe him to Saul. Everything in the message could have been deduced as a good guess by anyone with a decent knowledge of the current political situation. This passage reads like like a classic description of a séance, a deception that hangs entirely upon the customer's willingness to believe.

The important thing to note about the story is that Saul 1) had just cast-out the mediums from the land, and 2) that Yahweh would not answer Saul "in dreams" etc. In other words, Saul's actions should be viewed as both hypocritical and circumventing Yahweh--In other words, it's showing Saul's depravity and separation from Yahweh.

As far as the actual seance is concerned, we, as the readers, have to decide what actually happened. While I myself am skeptical of how effectual seances are, etc., there was no such skepticism in most of the ancient world--including ancient Israel. I don't think there is any reason to suspect that the author thought it impossible that a medium could actually communicate with the dead. You might have a look at the bibliography in Brian Schmidt's Israel's Beneficent Dead if you are in to that sort of thing.

As far as Mike's comment is concerned, his statement that "Naturally there was a belief that holy disembodied souls lived in heaven" is unsupported by contemporary biblical scholarship. On the contrary, in the Hebrew Bible "heaven" is the abode of God. When humans die, they go to "Sheol" (the "grave" or "shadowy place") whence they shall be resurrected. You should check out Jon Levenson's book Resurrection and the Restoration of Israel if you want more evidence.

I believe it really was Samuel coming to confront Saul. Whether the medium knew that she was dealing with demons to deceive those who came to her, or really believed that she was bringing up the dead, this was something she had never encountered before. She did not have the power to bring Samuel, or anyone else, back from the "bosom of Abraham." (In the parable of the rich man and Lazarus, Abraham said someone returning from the dead would not convince the rich man's brothers, but never said someone could not return from the dead by God's power.) Mason points out that Saul could not see Samuel, but apparently they spoke to each oter (no mention is made of this conversation being conducted through the medium). Samuel makes prophecies that no one else could have done (the death of Saul and his 3 sons, and the defeat of Israel's army).

It was Satan or a demon impersonating Samuel

God was not answering Saul because of his disobedience, "neither by dreams, nor by Urim, nor by prophets" 1 Samuel 28:6. Saul becoming desperate, turn from God to obtain light from an agent of Satan.

By going to a witch, Saul has placed himself in the enemy's territory. Through the Bible we also know that Satan is fully capable of impersonation.

It was not God's holy prophet that came forth at the spell of a sorcerer's incantation. Samuel is not present in that haunt of evil spirits. That supernatural appearance was produced solely by the power of Satan. (Patriarch and Prophets, by Ellen White, Pg 679)

The woman's first words under the spell of her incantations had been addressed to the king, "Why hast thou deceived me? For thou art Saul." Thus the first act of the evil spirit which personated the prophet was to communicate secretly with this wicked woman (Ibid, Pg 680)

Satan, by his bewitching power, had led Saul to justify himself in defiance of Samuel's reproofs and warning. But now, in his extremity, he turned upon him, presenting the enormity of his sin and the hopelessness of pardon, that he might goad him to desperation. Nothing could have been better chosen to destroy his courage and confuse his judgment, or to drive him to despair and self-destruction. (Ibid, Pg 680)

It is interesting to note that when God rebukes or pronounces judgement, invitations of repentance are usually also accompanied through the Holy Spirit, as evidenced in Nineveh (because repentance is from God (2 Tim 2:25)). The people just may not accept it. However, when mercy is run out, God usually is silent, in the case of Jesus not uttering a word to Caiaphas and others before His cruxifixction.