And there followed him a certain young man, having a linen cloth cast about his naked body; and the young men laid hold on him: And he left the linen cloth, and fled from them naked. (Mark 14:51-52 KJV)
User
The Bible does not teach us who he was, so of course we do not know.
One hypothesis is that he was the author of the Gospel - that is, he was Mark. The only evidence for that is this: only that Gospel mentions that person. One can suppose (or propose) that only the person who experienced that event would consider that particular event important enough to include in the Gospel.
Anonymous
it's believed to be the author, Mark.
John spoke of himself also in the third person.
it's assumed to not draw attention to themselves.
Anonymous
It was Ash looking for Pikachu.