Sayaka shows up in just a single episode, but she leaves a surprising mark on the series—she's essentially the reason Yusuke and Keiko actually get together. Koenma sends her to investigate Yusuke, and during that mission, she ends up testing whether his feelings for Keiko are genuine.
In the manga version, Sayaka's backstory is sadder. She's the lonely spirit of a girl whose parents never knew how to care for her properly. To cope with that isolation, she kept pulling Shota's soul out of his body—and since he had no spiritual awareness, it was basically draining his life away bit by bit.
Yusuke and Botan manage to get through to her, making her realize what she's doing. Once Shota promises to spend time with her in Heaven eventually, she finally lets him go and stops the haunting. After that, she hangs around with Yusuke for a while, using the time to figure out whether he and Keiko actually care about each other or if it's just surface-level attraction.
Content compiled by AnimeList.moe from publicly available sources.
