The Skinless Boy is a Spirit-Eater created by Superior Saturday using Arthurs pocket. It took the place of Arthur on Earth, and spread the virus known as Greyspot. It was killed when Arthur threw the pocket that bound it to life onto a spike of Nothing.
Grim Tuesday: The Skinless Boy is seen briefly at the end, where it looks at the wall Arthur has built. Also, in the prologue, two strangers show the Grim the pocket and urge him to destroy it. The Grim forgets it however, and it is stolen by Saturday's spies.
Drowned Wednesday: In the very last chapter it is shown to have taken Arthur’s place on Earth.
Sir Thursday: The Skinless Boy has a prominent role in this book. Arthur is not allowed to fight it, because such a collision would destroy mankind. It starts spreading the virus Greyspot, (a virus so named because it leaves grey spots on whatever parts of the body the Skinless Boy touches) a virus that renders anyone who suffers it under its control. Leaf steals the pocket that binds it to life, but in the process catches Greyspot. She eventually manages to give the pocket to Suzy, who in turn gives it to Arthur. Arthur throws the pocket onto a whirling spike of Nothing, destroying the Skinless Boy and the Greyspot virus in the same instant.
The Skinless Boy's appearance is very strange, because one half of it is human, and the other is shapeless flesh (hence "Skinless Boy").