Satella is described as a half-human, half-elf woman with long silver hair and amethyst eyes that shimmer with a bluish tint. As a half-elf, she has pointed ears, and her beauty is both captivating and otherworldly. When she manifests, her body is often concealed by dark mist so deep that it swallows even the surrounding darkness, leaving only her glowing eyes visible.

Beneath the shadows, Satella wears black thigh-high socks with orange frills, a black one-piece dress adorned with orange flowers, and a long dark veil that partly hides her face. She also wears a pair of earrings and an orange-and-black eight-pointed star hairpiece.

When she appeared through Emilia’s body, her form was once again shrouded in shadow, her voice distorted beyond recognition. Later, during Subaru’s encounter with her in Echidna’s tea party, she wore a dress of darkness that pulsed with her heartbeat. Initially, Subaru could not see her face due to his own denial, but when he finally accepted her, he saw that Satella looked exactly like Emilia.

Contrary to popular belief, Satella and the Witch of Envy are separate personalities born when she absorbed an incompatible Witch Factor. The two constantly struggle for control. Both love Subaru, but the Witch of Envy’s affection is obsessive and violent, while Satella’s is gentle and selfless. Their shared love for Subaru is the reason he was granted Return by Death, to keep him from truly dying.

Before her imprisonment, the Witch of Envy was starved for love. For reasons unknown, she became fixated on Subaru, endlessly repeating “I love you” in her shadowed realm. Her obsession made her extremely possessive, silencing Subaru whenever he tried to reveal Return by Death and killing anyone who learned of it. Her wrath was also seen when she violently expelled Petelgeuse from their realm for intruding. Though normally indifferent to others, she was enraged when Subaru claimed to prefer the other Witches over her.

Alec Hoshin described Satella as earnest, reckless, pure, and honest despite her cursed fate. The author noted she lacks all sense of common sense. When appearing in the Castle of Dreams as Satella rather than the Witch of Envy, her true kindness emerged, she wept for Subaru’s suffering, showed deep concern for him, and gently listened to his words. Unlike her darker half, Satella is calm, compassionate, and ultimately asks Subaru to one day end her life.

Satella possesses an array of overwhelming powers that make her one of the most dangerous beings in existence. Her primary ability, the Authority of Envy, remains largely undefined, though it is suggested to remotely influence others through jealousy. Her most terrifying trait is her flawless immortality, as her body cannot be destroyed by any known means, a fact that even Flugel, Reid Astrea, and the Divine Dragon Volcanica failed to overcome, forcing them to seal her instead. She also demonstrates mastery over shadows, using them as a weaponized miasma capable of consuming life, matter, thoughts, and memories. Through shadow tendrils, she captures and annihilates her enemies, even destroying them from within. Her devouring shadows can engulf entire regions, while her veil of darkness nullifies all physical attacks. Satella can also manifest countless shadow hands, which resemble Petelgeuse’s Unseen Hands but are much stronger, and she uses them to grip Subaru’s heart whenever he attempts to reveal Return by Death. Her influence extends beyond physical combat, as she can manipulate emotions, distort space, and possibly even alter time, halting it when Subaru discusses his secret.

Aside from her Authorities, Satella is a master of Yin Magic, able to command shadows and teleportation-like spells. Her scent, known as the Witch’s Scent, is so potent it warps perception, destroys flesh, and corrupts souls. She is also capable of possessing others, as seen when she took control of Emilia’s body and later partially possessed Subaru himself, unleashing waves of shadow linked to his emotions. In her prime, Satella’s strength eclipsed all other Witches, swallowing half the world and nearly defeating the combined might of the greatest heroes of her time. Even four centuries later, she remains nearly unrivaled, matched only by the Sword Saints Reid and Reinhard van Astrea. Though sealed within the Evil Sealing Stone Shrine, her immense power leaks beyond its limits, and in Arc 9, she managed to manifest outside her prison once more, proving that even confinement cannot completely restrain the Witch of Envy.

Before the Great Calamity, Satella journeyed with Echidna, Flugel, Reid Astrea, Farsale Lugunica, and Alec Hoshin for unknown reasons, though Alec described their mission as an attempt to save the world. During their travels, they kept their identities secret, but when they arrived in the nation of Magrizza, Farsale revealed his name, drawing the attention of the royal family. The crown prince, captivated by Echidna, secretly arranged a marriage with her, though her companions remained unaware. However, before the union could happen, Reid and Volcanica went on a rampage for unexplained reasons, forcing the group to flee. Soon after, Magrizza was annihilated by the Black Serpent, which Echidna believed had been pursuing them.

Four centuries ago, Satella took in the Witch Factor of Envy, which was incompatible with her, creating the separate and destructive Witch of Envy personality. Though this did not directly cause her rampage, Satella lost control and nearly destroyed the world, killing countless people. Despite the combined efforts of the Divine Dragon Volcanica, the first Sword Saint Reid, and the Sage Flugel, she could not be slain and was instead sealed within a shrine guarded by Shaula until the events of Arc 6. The site later became the Pleiades Watchtower and the Evil Sealing Stone Shrine, now watched over by Volcanica. Even while sealed near the Great Waterfall, Satella maintains a mysterious link to Subaru, remaining aware of his every action and manifesting as a shadowy figure that appears to protect or observe him throughout the story.