Rider

Heroic Spirit of the mount. A type of Servant that requires high maneuverability, and tends to possess strong Noble Phantasms.
The Rider in the story is no exception, riding Pegasus and making full use of its charging power in battle.
Of course, Rider herself is also a unique Heroic Spirit, and the number of skills and Noble Phantasms she has is particularly high even amongst the Servants.
She is taciturn and lacks self-assertiveness, but that doesn't mean she is apathetic toward her surroundings. Rather, she's the kind of person that worries about things from every conceivable angle but hides it behind a perfect poker face.
Though parts of her personality are quite similar to Sakura's, Sakura is a commoner while Rider is more of a high-born type. Even her demeanor and tone of speech have a sort of high-class strictness to them.
With the complex she has about her own height and the way she gets embarrassed when people see her mystic eyes, she might actually be pretty lady-like.
Because of her outward appearance and primary weapon, a lot of people thought she was going to be an Assassin class Servant before the game came out, but nope, she's a Rider!
I really wanted to have a scene in the Sakura route where she and Saber bickered with each other as they fought toward a common goal, but unfortunately the story didn't develop in that direction and I had to discard the idea.
Incidentally, the lewd dream on the fifth day of the Sakura route was Rider's doing.
She is not a Dead Apostle (vampire), but she is a bloodsucker. For her, the most efficient, luscious way of gaining magical energy is drinking blood.

Fate/side material: Fate Encyclopedia