Assassin of Red

One of the Servants and masterminds of the “Red” camp. A peerless beauty who wears a black dress. She refers to herself quite haughtily, but she is actually a comparatively straightforward person.
Her true name is Semiramis. The legendary empress who is called the world's oldest poisoner. Though her class is Assassin, she actually has the traits and abilities of the Caster class as well. She likes decadent luxury, but that does not mean that she hates the simple things in life. In this Greater Holy Grail War, she set out to bring about the realization of the Third Magic along with Shirou Kotomine.
The rank of her Agility and Presence Concealment is low for an Assassin, but, thanks to her Noble Phantasm “The Hanging Gardens of Babylon: Aerial Gardens of Vanity,” her magical attack boasts of enough power to make someone like Caster of “Red” run away barefooted. Also, just as its name suggests, the Hanging Gardens has the special ability to move while floating in mid-air and it can also attack while maintaining a strong defense.
However, in a normal Holy Grail War, Semiramis would be in an extremely disadvantageous position. Since her Noble Phantasm is so huge, it is practically impossible for her to conceal her presence. Also, some very difficult conditions must be met before her Noble Phantasm can be activated in the first place. Furthermore, even if she managed to activate the Hanging Gardens after much hard work, she would not be able to defend the Gardens by herself. No matter how much peerless power she boasts of, she will always be at a disadvantage when fighting alone. A Greater Holy Grail War like this might be precisely one of the few chances that allow her to fully demonstrate her specialties.
Like Atalanta, she was abandoned by her mother as a child. In her case, it is said that she was warmly embraced and raised by doves, perhaps due to her being a demi-god. Afterwards, she went on to marry the old general Onnes, but she was forcefully stolen away by King Ninus, who was enraptured by her beautiful face. A few days after Semiramis became empress, which she suggested to King Ninus in order to deepen her affection for him, she killed him with poison—or so it is said.
What lies at her core is an absolute rejection towards others. She doesn't like solitude, but as someone who prefers being aloof and superior, all humans have to be “people who unquestionably serve her” in her eyes. The reason all the Servants who should nominally be her allies hate her is because they intuitively sensed her way of thinking.
…However, due to the man who is both a priest and a Servant and who, despite being “someone who walks alongside her” instead of “someone who serves her,” held not the slightest interest in Semiramis herself, her thoughts and feelings became disordered and fell to pieces.
This is because this was the first time for Semiramis that someone valued her not for her “beauty,” but rather for her “abilities.” At first, that attitude towards her made her feel good, then she felt a strange anger, and finally her chest was pained by a feeling that she tasted for the first time since she was born—a feeling that could be called heartache.
In the end, the kind of person she truly liked was someone who moved forward without looking back at her at all… It was already too late by the time she realized that, though.
“When the Third Magic is realized, you will become the queen who rules the entire world”… is what Shirou told her, but in truth, she wasn't that obsessed with ruling the world. What she truly sought and wished—was something that she couldn't understand even at the brink of death.
Perhaps what this poisonous woman sought was something like pure water.

Fate/Apocrypha material: Fate/Apocrypha Encylopedia