Sayaka is the person who was sent by Koenma to do an investigation on Yusuke. If you think about it, Sayaka was the one who sparked Yusuke and Keiko's relationship. The only time she is ever seen is in one episode.
In the manga, Sayaka is a spirit of a lonely girl who's parents didn't know how to take care of her. In her loneliness she kept taking the soul of Shota, and because Shota didn't have any sixth sense, it was slowly killing him.
Fortunately, Yususke and Botan managed to knock some sense in her and she left Shota alone after he promised to play with her in Heaven when he gets there. She then hangs out with Yususke, and during that time she tests his and Keiko's love for each other.
Vova
