Mark was a villager who at some point gained access to the Emerald Dimension and turned into an emerald block.
It is unknown what Mark looked like as a villager, but eventually, he turned into an emerald block, indistinguishable from any other emerald block.
He is very serious about the Villager Gods. He was at one point a successful businessman who gained access to the Emerald Dimension by the Villager Boss.
If an Emerald Dimension was Added
As Villager #2140093 was trying to escape the dimension, Mark knocked him out unconscious. He brought him to a ritual with the rest of his cult and explained to him what is happening. As he was explaining, Villager #2140093 made his escape, but Mark continued talking, until one of his fellows told him that he was talking to nobody.
- Mark and the rest of Emerald Blocks could be actually retextured Slimes, since they've got a full block shape.