She is summoned as a female due to stories of him cross-dressing at homosexual parties. She has raven black hair and dark blue eyes, her bangs covering her right eye. She is initially shown wearing a black lingerie gown, but is also shown to wear a brown suit with an overcoat, red tie, gloves, and a hat. She's also shown frequently with a cigar.
Assassin has been shown largely to be manipulative and conniving, aiming to control the actions of the other Servants into destroying themselves, however her plots are often foiled to comedic effect, such as Gudako bursting in on her for badmouthing Paul Bunyan. She is also indicated to be extremely proud and boastful, even by her own admission, believing that surviving the War to the end is the only way the Servants can get implemented into Fate/Grand Order. Despite her pride, she is shown to have a weakness in her Mother's approval. She considers her Master as a Maternal figure, trying to dissuade her efforts to expose what Assassin knew by claiming that she was her "Mama" rather than her "Mother", but failing and exposing the location of Gudako upon Olga expressing disappointment, claiming she would not expect anything from her anymore.
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She is shown wielding two different pistols, when wearing her alternate outfits, and has claimed to wiretap every panel of the Manga. Her personal skills are Filing, Coercion, and Espionage. Her Noble Phantasms are Official and Confidential and Government Men.
Matsuryu, artist of Helena Blavatsky, posted a fan art mockup card of the character on Twitter, characterizing her as Avenger Yoshiko Kawashima. As it was posted the day before the anniversary of the Nanking Massacre, some people took offense to the characterization, causing Matsuryu to apologize and remove the tweet.