Sayaka

Sayaka is the individual Koenma dispatched to investigate Yusuke. Upon reflection, Sayaka was the catalyst for the relationship between Yusuke and Keiko. She appears in only a single episode.
In the manga, Sayaka is the spirit of a lonely girl whose parents did not know how to care for her. In her isolation, she repeatedly took Shota's soul, and because Shota lacked any sixth sense, this was slowly killing him.
Fortunately, Yusuke and Botan managed to reason with her, and she left Shota alone after he promised to play with her in Heaven once he arrived there. She then spends time with Yusuke, and during that period, she tests his and Keiko's love for each other.