Fatal Error is a powerful and malicious AI hiding within Sonic's body with the goal of consuming code to the point of destroying various programs.
Fatal appears as a giant red and black monster hiding within a Sonic body. He has small sharp red teeth with four giant arms with red clawed hands. His true form can be seen peeking in his black torso in the form of a small eye blinking out. He has red shoes with black bands and a small swirling tail.
Fatal Error is also British for some reason.