Creep originally was a Demon of Gap and helped other Demons. He assisted Exetior during the first war. When Exetior lost, Creep was punished and forced to serve mortals. Exetior doesn't remember Creep, but Creep remembers him.
Creep first appears when Exetior enters Sonic's mind when he is sleeping in Green Hill in an attempt to possess his body after having escaped from prison.
It seems to work as first, until "Sonic" begins to restrain the arch-demon before Creep reveals himself to be the "dark guardian" of Sonic's mind and is stopping Exetior from trying to take his body and claims he won't simply let him take it so easily.
Exetior is angered that he is being denied Sonic's body, but Creep tells him that he can't take Sonic's body due to Sonic possessing a very strong will. However, by killing his friends, he can destroy Sonic's hope and destroy his spirit, allowing him to take his body with ease, before sending Exetior away.
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Creep only appears in a secret involving Sonic in his mind.
In the Ex-aitor ending, when Exetior is walking in Sonic's mind, he sees Creep's mask on the floor. When Exetior asks Sonic where Creep is, Creep answers him and warns Exetior that it's a trap and he should get out. However, he is not seen after that.
When Sally gets possessed, he appears to tell her that she's been possessed. He then goes to check on Sonic, and asks him who is there, to which Sonic replies "Nobody". Sally follows Creep, and then he informs her of the details of what is happening to her. He also tells her that she should join Exetior, but Sally doesn't believe him. Creep then tells Sally to return to her mind and prevent the Highest from getting to Exetior, and she agrees. Creep then disappears.
Creep is responsible for Sonic's mind and stops others from taking it over. However, he does not show that much concern for Sonic's well-being, as he is quick to tell Exetior how to destroy Sonic's spirit. He is loyal to Exetior.
Creep has blue fur, light skin, white gloves, and red shoes like Sonic, but his face is completely black and nearly featureless with only red glowing eyes. He has only three spines.
Creep almost always wears a grey mask with a humanoid face with black eyes and a black mouth that stretches to the sides of the mask.
Creep can fly, and he also has the ability to enter Sonic's mind, being his guardian. He is additionally able to copy Sonic's appearance.