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Mahou Shoujo Madoka Magica character.
An armored mermaid witch (Madoka Magica) introduced in episode 9 and seen again in episode 10. Her minions are Holger (Madoka Magica) (the performer) and Klarissa (Madoka Magica) (the backup dancer).
Note: Major spoiler warning!
She is the tragic result of Miki Sayaka becoming a Witch.
Also becomes a stand-like entity for Sayaka in the Rebellion movie.