Satella is the Witch of Envy and the one responsible for the Great Calamity that took place 400 years before Subaru's arrival. She is currently sealed off in the Evil Sealing Stone Shrine in the Augria Sand Dunes.
Satella has a thin body with long silver hair and purple blue eyes.
When Subaru encountered her at Echidna's tea party, she wore a black dress made of shadows that seemed to wave in accordance with her heartbeat. Initially, he was unable to see her face because of a black shadow covering her above her neck, which was created out of his reluctance to accept her, however he later accepts her and becomes able to see her face, mentioning afterward that she looked exactly the same as Emilia.
Contrary to popular belief, Satella and the Witch of Envy are separate personalities, formed when she took in incompatible Witch Factors. The two personalities tend to fight each other. Both personalities love Subaru, though the Witch of Envy personality is much more aggressive about it, while Satella is much more reasonable and cares deeply about his well-being.