Echidna’s barely exposed skin was porcelain-white, so pale that it seemed devoid of any trace of color. Her black eyes, filled with intellect, were the only feature that appeared truly alive. Her long, straight white hair flowed down her back, adorned with a black butterfly hairpin (jade green in the anime). Her beauty was mesmerizing, yet her presence instilled a deep, instinctive fear in all who saw her. Though her spirit in the Castle of Dreams appeared in her late teens, her slender physique resembled that of a woman in her mid-twenties.

Only her head, neck, and hands were visible, the rest of her figure concealed beneath black garments. The stark contrast between her pale skin and dark clothing accentuated her striking elegance. During her meeting with Subaru in the Trial of the Past, Echidna appeared in an outfit drawn from his memories, a female version of his high school uniform. This ensemble included a white dress shirt, a navy blue blazer bearing the school crest, a red ribbon at the collar, a grey pleated mini skirt, knee high white socks, and white uwabaki slippers with brown soles. She replaced her usual hair ornament with a small red butterfly clip.

Despite her divine beauty, Echidna exuded a miasma of fear so intense that most people would vomit upon encountering her. Prolonged exposure to her presence was lethal, though Subaru, while unaffected physically, noted that her aura felt as dangerous as, if not greater than, that of the White Whale or the Sin Archbishop of Sloth.

The Echidna from four centuries ago and the Echidna within the Castle of Dreams were stated to be distinct entities. While their appearances were nearly identical, the former appeared to be in her mid to late twenties and possessed a calmer, wiser expression. Echidna was often seen smiling serenely and behaving with composure, though her demeanor would shift to one of intense fascination when discussing topics that piqued her curiosity. Her beauty and grace gave her a maidenly, almost ethereal impression, concealing the vast intellect and ambition beneath her gentle exterior.

Echidna’s boundless curiosity and thirst for knowledge defined her entire being. She referred to this obsession as love, directing it toward Subaru’s Return by Death, which fascinated her to the point that she wished to become his advisor and record the countless possibilities his power created. On the surface, she appeared polite, soft spoken, understanding, and at times childlike. According to the other Witches, she was also clumsy and forgetful, once nearly burning herself alive after accidentally setting a forest on fire while trying to keep warm. She also once went out in public wearing only her nightgown, having forgotten to change.

Despite her composed and courteous demeanor, Echidna was described as black hearted and incapable of understanding the emotions of others, displaying traits akin to a sociopath by normal standards. She claimed to have learned only negative emotions such as contempt and ridicule from her parents and admitted she could not comprehend true love or kindness due to never having experienced them. In her pursuit of knowledge, Echidna had no moral restraint, freely using lies, manipulation, and deception to achieve her goals.

Though she treated Subaru with politeness and warmth, her charm concealed a calculating nature. Even after Subaru called her a monster, she maintained her calm and friendly facade. When he rejected her offer of partnership, Echidna respected his decision but revoked his qualification to enter the Castle of Dreams, doing so as subtle revenge. She showed genuine hostility only toward Emilia, Satella, and one unnamed individual. While she often acted detached from emotion, Echidna still retained traces of the personality she revealed to Subaru, suggesting her feelings were not entirely feigned.

As the Echidna from four centuries ago and the Echidna from the Castle of Dreams were separate entities, their personalities also differed. The original Echidna was noted to be more considerate, empathetic, and helpful than her counterpart within the Castle of Dreams.

Authority of Greed: Echidna possessed the Authority of Greed, granting her the ability to perceive all events in the past, present, and future. This power was later lost after she returned as Omega. The Book of Wisdom used by Beatrice was an imperfect copy of this ability, containing fragments of the world’s memory and immense knowledge that could overwhelm ordinary minds.

Magic: Echidna was a master of all six magical elements and created numerous original spells. Her mastery was unmatched until Roswaal L. Mathers surpassed her. She could perform high-level, multi-fold magic capable of piercing even Volcanica’s defenses.

Miasma: Her presence emitted a deadly miasma that induced nausea, madness, or death. Omega weaponized this power, killing enemies instantly.

Soul Transcription: A lost magic she created to achieve immortality by transferring her soul into another body. Though unfinished in life, it was later perfected by Roswaal and eventually used by Echidna to possess Ryuzu Shima’s body, becoming Omega.

Dream Arts & Castle of Dreams: Echidna used Dream Arts to create the Castle of Dreams, a space beyond the real world where the souls of the Witches dwell. She could freely alter its form and control entry and exit portals.

Memory Manipulation: She could view, alter, and erase memories, as shown when she examined Subaru’s mind and suppressed his recollections of their meetings.

Echidna was the only Witch born as one, raised by parents who taught her only negative emotions, which led her to despise them. Centuries ago, before the Great Calamity, she traveled with Satella, Flugel, Reid Astrea, Farsale Lugunica, and Alec Hoshin on a secret journey to save the world. During this time, a failed incident in Magrizza involving a proposed royal marriage and Reid’s rampage led to the nation’s destruction by the Black Serpent.

Over four hundred years ago, Echidna saved a young Roswaal from mana poisoning, became his mentor, and created three artificial spirits, Puck, Beatrice, and one other, while researching immortality through soul transfer. She founded the Sanctuary to protect her experiments and fought Hector, the Warlock of Melancholy, after Ryuzu Meyer’s sacrifice activated the barrier.

During her studies, Echidna created clones of Ryuzu, one named Sphinx, whom she later abandoned. She was sought by royals across nations for her wisdom, helped found Kararagi, and became acquainted with Pandora, the Divine Dragon, and Emilia’s mother. She also mentored Aldebaran and once employed Petelgeuse to guide Beatrice.

After the deaths of the other Witches of Sin, Echidna gathered their souls and was later sealed within her tomb in the Sanctuary by Volcanica as a deterrent to Satella.