Mark 16:5
And entering the tomb, they saw a young man sitting at the right, wearing a white robe; and they were amazed.
The common interpretation is that this is an angel. I’ve also heard that this could be an unidentified witness.
My question is: What evidence do we have, hermeneutical or otherwise, for who this young man was?
We have a simple comparison of the gospel accounts:
2 Suddenly there was a great earthquake, for an angel of the Lord descended from heaven, rolled away the stone, and sat on it. 3 His appearance was like lightning, and his clothes were white as snow. 4The guards trembled in fear of him and became like dead men.
5 But the angel said to the women, “Do not be afraid, for I know that you are looking for Jesus, who was crucified. 6 He is not here; He has risen, just as He said! Come, see the place where He lay. 7 Then go quickly and tell His disciples, ‘He has risen from the dead and is going ahead of you into Galilee. There you will see Him.’ See, I have told you.”
5 When they entered the tomb, they saw a young man dressed in a white robe sitting on the right side, and they were alarmed. 6 But he said to them, “Do not be alarmed. You are looking for Jesus the Nazarene, who was crucified. He has risen! He is not here! See the place where they put Him. 7 But go, tell His disciples and Peter, ‘He is going ahead of you into Galilee. There you will see Him, just as He told you.’ ”
4 While they were puzzling over this, suddenly two men in radiant apparel stood beside them.
5 As the women bowed their faces to the ground in terror, the two men asked them, “Why do you look for the living among the dead? 6 He is not here; He has risen! Remember how He told you while He was still in Galilee: ...
John does not record anything about the women's conversation at the tomb.
We observe several things about the record as we have it:
The description of the angels at the tomb as "men" is common in Scripture as we see the same thing in other places as well such as:
Regarding the biblical evidence, @Dottard's answer is more than adequate. Among apocryphal testimonies, perhaps the most interesting is found in the Gospel of Peter. In this narrative Pilate sets soldiers to safeguard the tomb at the behest of Jewish leaders, fearing a miracle or a popular uprising. The young man of Mark 16 appears vs. 55. His clearly angelic counterparts are seen in vss. 36-40. They are gigantic, and the risen Christ's head reaches beyond the heavens:
[35] But in the night in which the Lord's day dawned, when the soldiers were safeguarding it two by two in every watch, there was a loud voice in heaven; [36] and they saw that the heavens were opened and that two males who had much radiance had come down from there and come near the sepulcher... [39] And while they were relating what they had seen, again they see three males who have come out from they sepulcher, with the two supporting the other one, and a cross following them, [40] and the head of the two reaching unto heaven, but that of the one being led out by a hand by them going beyond the heavens...
[50] Now at the dawn of the Lord's Day Mary Magdalene, a female disciple of the Lord... [51] having taken with her women friends, came to the tomb where he had been placed.... [55] They found the sepulcher opened. And having come forward, they bent down there and saw there a certain young man seated in the middle of the sepulcher, comely and clothed with a splendid robe, who said to them: [56] 'Why have you come? Whom do you seek? Not that one who was crucified? He is risen and gone away. But if you do not believe, bend down and see the place where he lay, because he is not here. For he is risen and gone away to there whence he was sent.' [57] Then the women fled frightened.
Here the seemingly human young man stands in stark contrast to the supernatural beings who must certainly be angels. However, since scripture frequently refers to angels as "men" this does not constitute evidence that the young man is not an angel. And of course, the Gospel of Peter is not canonical. It appears to have combined two or more accounts and exaggerated them.
