Ky, literally "princess", refers to the "key" for the "Lid of the Kettle of Hell". The Ky controls the opening of the Lid and can be used to seal collected jaki under the Lid, away from this world.
The Ky is a person, who can be either female or male (despite the kanji's literal reading of "princess"), and may or may not have prior knowledge of sorcery or other mystic arts. When the current Ky dies, somebody living in the town where the "Lid" is located automatically becomes the new Ky. The new Ky will just learn of his/her new status and responsibilities without being told by anybody else.
For a list of Kys, see Category:Kys (spoiler!)
- Under typical conventions on the wiki, Ky would have been translated as "Ki" or "Princess". It is translated as "Ky" here as an alternative way to portray the unusualness of the written character vs pronunciation