Dean/Castiel
From Super-wiki
Dean/Castiel slash fanfiction pairs the characters Dean Winchester and Castiel in a romantic or sexual relationship.
Brief Overview
From Castiel's first appearance in 4.01 Lazarus Rising, fandom took the on-screen chemistry between Dean and Castiel and took it further. It soon became a very popualr pairing, probably partly becuase slash writers who were not into the pairing of Sam/Dean had few other outlets on the show.