The Celestial Spirit King (星霊王 Seirei-ō) who rules all of the spirit world and enforces the rules among all the spirits. He is the most powerful of all the spirits although it is unknown if he gives out his key to humans or even if he has one.

(Source: http://fairytail.wikia.com/wiki/Celestial_Spirits#Celestial_Spirit_King</a>)