Yamime The Dark Deity a.k.a The Dark Eye is an evil entity that possessed the body of Shinobu Matsuba, Satsuki's

The Dark Eye

'new' classmate in Episode 15. She appears at Satsuki's school after a ritual is held by a student at night after everyone had gone home and attacks the girl. Yamime, posing as Shinobu Matsuda using her stolen body, is introduced as Satsuki's new classmate. She has knowledge of black magic and convinces a group of girls (including Satsuki) to perform another ritual, saying that if they do, they could have anything they desire.
Shinobu, episode 15, timestamp 13-19

Shinobu, as she first appears to the girls

The ritual was actually a trap for Yamime to steal the souls of the girls, and erase them from existence. After Satsuki discovers the plan, Yamime appears at her house while her father and brother are asleep to attack her. Satsuki consults the Ghost Diary and is told to "bind her with red string and bathe her in sunlight." Satsuki finds she has no red string and instead draws red lines with lipstick on her closet mirror in place of string so when Yamime looks into it, it appears as if she were tied up. To 'bathe her in sunlight', she uses a camera flash on Yamime, putting her to spiritual sleep. The next day, the girls who had been erased returned.

Yamime, episode 15 timestamp 19-35

Yamime, stunned briefly by Satsuki's "red string".

It is revealed that Shinobu was a classmate of Satsuki's mother who had gone missing under mysterious circumstances.

Yamime appears briefly in Episode 20 after Ouma resurrected her.

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