Mark 14:51

“And there followed him a certain young man, etc..] Some think this was John, the beloved disciple, and the youngest of the disciples; others, that it was James, the brother of our Lord; but he does not seem to be any of the disciples of Christ, since he is manifestly distinguished from them, who all forsook him and fled: some have thought, that he was a young man of the house, where Christ and his disciples ate their passover; who had followed him to the garden, and still followed him, to see what would be the issue of things: but it seems most likely, that he was one that lived in an house in Gethsemane, or in or near the garden; who being awaked out of sleep with the noise of a band of…”

“See the notes at Matthew 26:47-57. Mark 14:45 Master, Master - As if expressing great joy that he had found him again. Mark 14:51 A certain young man - Who this was we have no means of determining, but it seems not improbable that he may have been the owner of the garden, and that he may have had an understanding with Jesus that he should visit it for retirement when he withdrew from the city. That he was not one of the apostles is clear. It is probable that be was roused from sleep by the noise made by the rabble, and came to render any aid in his power in quelling the disturbance. It is not known why this circumstance is recorded by Mark. It is omitted by all the other evangelists. It may…”

“A young man - It does not appear, that he was one of Christ's disciples. Probably hearing an unusual noise, he started up out of his bed, not far from the garden, and ran out with only the sheet about him, to see what was the matter. And the young men laid hold on him - Who was only suspected to be Christ's disciple: but could not touch them who really were so. Mark 14:53”

“14:51-52 The account of Jesus’ arrest ends with this comment about an unnamed young man. There does not seem to be any theological reason for Mark to record this incident. The explanation that it is an autobiographical detail about Mark is as good as any.”