Here is my idea.
This SCP is a man that exists yet NOT exists both at the same time. he phases in an out, sorta like a living .gif image. he seems sorta transparent and blinks in and out of reality both at the same time.
It is hard to explain how quick he phases in and out. Imaging cutting a second in half, then cutting one half of the second into more halves, and repeating this twelve more times. The resulting half would be how fast he phases in and out of reality. This is so fast that he looks like a transparent blur when he walks.
Also, he floats and moves through walls.
The foundation does not know why he does this or how he came to be, but they are working on the fact of containing him. He cant move through certain metals as fast as he can others. The stronger the metal is, he quicker he can move through it.
The man is named Billy S. Newton, as he claims.
(Here come that fellow!)