Stan is a fusion made when a badly summoned Xirnox had to bind with a host, who in turn casted The Rapening upon our multiverse and existance.
There are many Stan Fragments and one Stan Prime in the post-rapening existance.
He is all powerful with limitless control over the very existance of the cut-off Rapening multi-universe.
Stan takes the form of a 2d picture of Cody's face, it is unknown if this can even be changed or if is preferred.
The Bible of Stan
Someone out there chronicled how existence got like this named 'The Bible of Stan'.
Although it has one and only chapter it has many notes that clue in to how things work after the facts happened.