Mod

Mod is the main antagonist of Limbo. He was responsible for all the damage within the game world, and why it broke in the first place. Alongside that, he is actually not native to the game.

Mod himself is an incredibly savage creature. He seeks to sustain himself, even at the cost of the world around him. He cares little about what he is eating or how it will affect Limbo. On top of that, he seems to have an endless hunger, almost always munching on something. While Dex may view him as a massive threat to the system, Mod only sees Dex as a food supplier for the most part, only seeing the programmer as a valid threat when he himself is directly attacked by Dex.

So, how did he get into Limbo?
Mod was originally let into the game in an attempt to save a character (Presumably Glitch) from their brutal fate of the wolves. As such, Mod was only given the simple orders of "Destroy the wolves before they deal the fatal blow." He did just that, devouring the code of the wolves before Glitch was killed. However it was not before Glitch had suffered the loss of his arm. Even though Mod had completed his objective, he still wanted more: he was not satisfied. Soon he turned his attention elsewhere, and attacked the person he was meant to save, and devoured some of Glitch's code, before going on a rampage of eating. This mass devouring of the code was what became the Corruption, which awoke Dex from within the program and forced him to seal Mod under the game, thus ceasing the Corruption, but not until most of the game land had been eaten away. After that, Dex went to rebuild the game world, first salvaging the people who lived in the land by finding their remaining code and rebuilding it from there.

Basics:
25' long/???/Heavy/???

Other:
-While Mod was sealed below the game, it seems that seal is breaking, as he can pop up on the overworld from time to time, although just his upper body.