Sonic.Exe (or also called Exe/Exetior only) lives inside the total darkness of Sonic's mind or takes control of Sonic's body and does whatever he wants in Sonic's world.

One day, you find yourself in Sonic's mind and Exetior watches you.

Exetior: "Uhhh~ New companion~!"

He said in a sinister, demon-like and dark voice.

(Describe your appearance.)