Satella (サテラ) 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.
Personality
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.
History
400 years earlier, Satella absorbed Envy Witch Factors she had no compatibility with, resulting in the creation of her Witch of Envy personality. However, this alternate personality was not the reason for her subsequently going out of control and killing countless innocent people and nearly destroying the world. Despite the efforts of many, including the Dragon, the Sword Saint, and the Sage, Satella was unable to be killed and was instead sealed in a shrine that is watched over by Shaula.
Abilities
Authority of Envy (嫉妬の権能 Shitto no Ken'nō): Satella is able to use the Authority of Envy.
Although the abilities of the Authority of Envy haven't been revealed, in the Web Novel, Minerva mentioned during the Witches' Tea Party that Satella might direct her jealousy (妬み Netami) toward her if she stepped in between her and Subaru, suggesting that it's something that affects people remotely.
Shadow Control: Satella is able to control shadows, though it's unknown whether this is a part of her Authority of Envy. They erode or destroy all life and matter around her as well as taking abstract things like thoughts and sometimes even memories.
Satella is able to create shadow tendrils which she uses to capture, impale and decimate her enemies. The tendrils are strong enough to hold transformed Garfiel in the air and incapacitate him. She is also able to slip the shadows into someone's body through wounds and disperse them outwards, effectively destroying the opponent from the inside out. She can also create hands from her shadows. She can be seen gripping Subaru's heart, whenever he speaks about his ability to anyone who isn't her.
Time Manipulation: Satella is able to stop time every time when Subaru tries to tell someone about his Return by Death ability. However, it's unknown whether this is an authority or part of her Shadow Control.
Trivia
According to the author, the meaning behind Satella's name will be revealed in the final arc.
According to the author, if Satella and Reinhard van Astrea ever fought it would be a stalemate; Satella wouldn't be able to land a finishing blow whereas Reinhard can't bypass her immortality.
In the anime, Satella received a redesign by Ootsuka Shinichirou; likely due to her full appearance never having been drawn in any illustration or character sketch.
