Kagikko (鍵っ子)
A term used to refer to a "loyal" fan of the famous bishojo game company, Key (the creator of Kanon, AIR, CLANNAD, Planetarian, etc.).
Kagikko literally means "key child", it is a pun since it's also the Japanese equivalent of the western term
latchkey kid.
While other terms used to describe a devoted fan of a bishojo game company (such as 月厨 (Getchu, a TYPE-MOON fan)) are usually sarcastic or derogatory in meaning, Kagikko is generally not considered to be offensive. Although derivitive of the term exists, called Bakagikko (バ鍵っ子) (a combinition of baka (idiot) + kagikko), which is used to refer to ignorant or impolite kagikkos on discussion forums or BBSs.