|Also known as:||Unknown Girl (謎の少女, Nazo no Shōjo?)|
|Appears in:|| Capsule Servant|
|Character type:|| Divine Spirit|
Servant (Master: Protagonist)
Assassin's True Name is Stheno (ステンノ, Suten'no?), the first daughter of the Gorgon Sisters, the older sister of Euryale and Medusa, who were goddesses born from the wishes of mankind for ideal idols, the embodiment of the yearning hearts of men. While she and Euryale were "perfect goddesses" who would not age, Medusa had some kind of flaw that allowed her to grow. They should have been three united sisters, but this divide allowed them to become three separate individuals. While Medusa was different from the start, her existence also brought irregularities in the two who were still the same. Due to the jealously of Athena, Medusa was cursed, had the faith of her followers taken away, and was banished to the Shapeless Isle as a monster. She called Stheno and Euralye monsters because they would never grow old, but that was a lie to cover the fact that they were "perfected goddesses."
Out of love for their younger sister, they also decided to live on the island although they were still worshipped. Living peacefully for a time, Medusa fought off men wishing to slay the rumored monster and those who wished to take their goddesses away by force. Loving to toy with men, heroes seeking to court them were met with impossible demands and forced back to their home countries only able to weep. While those challenging Medusa were petrified, those seeking marriage were ridiculed beyond reason until meeting their deaths. Due to a deep hatred developing for the humans, Medusa soon began to travel down the path of a monster. Though they warned her not to rejoice in killing humans, she began to feast on their blood and pleasurably seek out new methods of killing them. As even they began to fear her, she soon left her sisters when she could not bear to been seen in her slowly transforming state, eventually becoming the Gorgon.
They eventually stood before the Gorgon as sacrifices after Medusa was completely gone, trembling while holding hands. Stheno lamented that they had been foolish in letting the situation lead to that state. She had not asked to be protected, but had wished for Medusa's happiness in protecting them to last. Euryale had also given up, but had to take some meaning in Medusa's protection lest Medusa "look silly." Deciding that the sister who protected them no longer being there meant that they had to disappear, they offered themselves to the Gorgon, holding hands in fear while not abandoning that which they held dear. Euryale bid her younger sister farewell, not able to help herself in letting her know that they were the ones who were loved by her. Though the past days would never return, she believed that the three of them would one day escape the temple and return home. She smiled like she had in the past and raised her head as Gorgon devoured them without emotion, absorbing the girls, "the sisters, who were beautiful to the very end," while still alive and having them become its flesh and blood without leaving a trace behind.
When Perseus later slayed the Gorgon after using Kibisis to reflect the nightmare of Breaker Gorgon back at the monster, the "nightmare" she had was that of her time with her sisters, causing her to be paralyzed with the joy of seeing them again as Perseus used Harpe to take off her head.
Stheno and Euryale are identical sisters called the "snow-white Gothic Lolita duo."
Stheno was pictured as the "ideal woman", displaying an elegant demeanor, an overflowing grace, and the wisdom in her actions and words. It is said that men will lose their senses over the rapturous feeling and swear eternal loyalty to her should just their names pass from her lips. Unlike their younger sister who is an expert in fighting and disliked humans, she and Euryale perfect themselves as idols who liked humans, welcoming the men visiting the island, comforting them, and sending them on their adventure. Though her image on the surface seems to paint that as the very meaning of her existence, that is only to those observing them from afar. She is a devil who loves everything pretty and cute, freely and unconditionally. While she loves humans, it is not as "living beings", but rather in the way of toying with those she likes, embarrassing and driving them to ruin while watching them struggle. In reality, they truly hate men, while their sister is the one who bares them no ill will. They, loved by men, should be destined to be violated by them day after day. Though she despised her fate, Stheno had given up until Medusa became the one to protect them.
Due to the "irregularities" caused by Medusa's growth, she would describe her older sister as the "perfect example of laziness." She has not the slightest interest in things she does not like. This demon-like sister's cruelty could make the watchdogs of hell tremble in fear. She loves to tease and belittle Medusa, but does deeply care about her. Along with her sister, they like to suck her blood, noting that it is extremely sweet. Due to remaining the same as her sister eternally, they have a closer link than even telepathy, like two parts of the same body who share the same senses, feel everything the other does, and share the basics of their personalities. They will often speak as if they are one person, using singular pronouns to describe each other.
Stheno appears in Capsule Servant as the High Priestess of the Dimensional Empire MATO (時空帝国マト, Jikū Teikoku Mato?). Alongside Euryale, they use the anti-SAKURA giant humanoid weapon MEDO-SA to battle Shirou.
Stheno and Euryale appear briefly in Carnival Phantasm Episode 12.
In Fate/kaleid liner PRISMA☆ILLYA 2wei!, the eleventh of the manga and the seventh episode of the anime, when Irisviel von Einzbern talks about the power of a big sister, Stheno makes a cameo appearance alongside Touko, Euryale and Kohaku to illustrate Irisviel's remarks.
Stheno is a Perfected Goddess (完成した女神, Kanseishita Megami?) with the power of immortality and eternal youth, differing from the gods of Olympus who had to halt their aging with the golden apples. Born as an "idol", her "divine essence" is to remain the same from the moment of creation to destruction. Despite that, she is of the weakest beings in the world. Possessing no ability to fight, she will starve if left alone, requiring the help of others to survive. Whereas other Servants may be constrained and reduced in ability by their class containers, Stheno actually becomes stronger when summoned as Assassin, simply because even the class container's base abilities are greater than hers. Differing from her sisters embodying the concept of "forever young and immortal", Medusa grew and developed abilities that allowed her to act as their protector. Her purpose was to be an "eternal idol", the incarnation of a pure maiden idealized as a never-aging young girl. Her purpose was "just to be loved and spoiled", whose beauty would never be matched by Medusa's best efforts.