In Mesopotamian mythology, Ereshkigal is the ruler of Irkalla, or the land of the dead. She is also Ishtar's sister, which is why she takes on the appearance of Toosaka Rin, but with blonde hair. Additionally, Ereshkigal's hair grows out in her Third Ascension.
Character Design: Morii Shizuki
Voice Actress: Ueda Kana