Lilith

Lilith (リリス) is a main character of Alive Renewal, Fake, and Rebirth. She also makes an appearance in Ichigo-da.

Background
Lilith is a shinigami who possesses a crystal connected to her life.

Alive Renewal
Lilith saves or revives Yomi with the help of her crystal, after which Yomi regards Lilith as her master.

Fake
Lilith takes on the alias "Sara Saeki" (佐伯 沙羅 Saeki Sara), becomes the wife of Yuugo Saeki, and takes on a job of a researcher at the Mikado Medical Research Institute. There, she offers to help Yuugo revive his deceased twin sister by using the Necronomicon.