Rusalka

Rusalka (ルサールカ, Rusaaruka) is a demon in the series.

History
In Slavic mythology, a rusalka (plural: rusalki) was a female ghost, water nymph, succubus or mermaid-like demon that dwelled in a waterway.

According to most traditions, the rusalki were fish-women, who lived at the bottom of rivers. In the middle of the night, they would walk out to the bank and dance in meadows. If they saw handsome men, they would fascinate them with songs and dancing, mesmerise them, then lead the person away to the river floor to their death.