NULL was a character created by Xanster, based off of their game Don't Leave Me.
NULL takes the form of the regular blue hedgehog, but has a gaping hole in his face and stomach. He also has bloody gloves.
Don't Leave Me is a story about Sonic 2, where Sonic and Tails go through all the different zones, but Tails seems to be figuring out that he's nothing more than life fodder. But still, Sonic wants to save Tails regardless.
NULL is based off of one of three endings in his game.

