Percy is a Blue Guy that is mentioned in Hippo Story 4 and My Adventure in Sammy's Blue World. He seems to be the leader of all the Blue Guys, and has not appeared in any books. It is possible his name comes from Percy from Thomas and Friends, or even Percy Jackson.
Hippo Story 4Edit
My Adventure in Sammy's Blue WorldEdit
He is mentioned indirectly by Chugs as "the leader".