I cannot believe this has no definition yet.
Anyway, Kafei is a character from The Legend of Zelda: Majora's Mask. He and his lover/fiancee, Anju, are the focal point of the Anju/Kafei Quest, which involves reuniting the two and is, quite honestly, the best sidequest in the entire game. If you successfully complete this task, at the end of the game, in the end credits, you will get a scene of Anju wearing her wedding dress and offering her hand to somebody (this occurs in first-prson view).
Kafei has become an extremely popular character and is renowned for being the first character that is not Link that you can play as. Also, many people associate Majora's Mask with Kafei (and Anju).
Kafei is one of my favorite Majora's Mask characters.