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

Rider is the Servant you fight in the first round.
Her Master is Shinji.
Other than pirates, who else would wear this kind of clothing but this female hero.
Her true name is Francis Drake, the great person who led the way in the Great Age of Navigation. According to historical records Francis Drake was a man, but in the world of EXTRA Drake is a woman.
In EXTRA she appears as a big-hearted villain, and she is the first obstacle that the protagonist needs to overcome. The protagonist gets one hell of a first experience at sea.
Francis Drake lived in England in the Great Age of Exploration. She was an adventurer and explorer, and captain of a privateer ship (pirate ship). She even eventually became the commander of a fleet of ships.
The first living person to circumnavigate the globe, the profit she made from sailing around the world helped England become a globally dominant force.
She also crushed Spain’s Invincible Armada. Spain was known as the “Empire where the sun never sets,” yet she effectively caused it to collapse by crushing that Armada. Hence she became know as the woman who knocked out the sun; Temerosse El Draque. She was certainly a Queen of the sea. (El Draque means devil or demon in Spanish)

She strongly believed in having lots of fun in life, and treats everyone equally regardless of whether they are good or evil.
She really loves living it up big and flashy. Whether it’s her personal life or battle, she wants to blow through it like a storm and enjoy the moment.
She loves treasure, but not the kind of treasure that is eternally valuable. She loves treasure that goes out with a bang like a firework.
“I got all these gold coins together just to spend them all in a flash, like a storm.”
Says her, holding a beer mug and and boasting like a exhilarating big sis.
(So in a way she does love gold, but doesn’t care for things like Gilgamesh’s inexhaustible treasure.)
She doesn’t dislike humans, but doesn’t love them either.
“Myself included, humans are just creatures who can’t be helped aye.” is how she feels about people who walk a vague line between good and evil. She’s always enjoying herself regardless of the situation.
So she has a broad-minded way of judging people.
She doesn’t hate her Master because he’s evil or because he betrays others. Shinji’s crooked elitism and other wretched acts are no problem as far as Rider is concerned.
She isn’t unsatisfied with herself as a Servant, and she isn’t disappointed that Shinji is her Master. Ever since she got the backing of the English Navy and started operating as part of the English fleet, she got used to regularly encountering incompetent officers of higher rank than herself, and incompetent aides. (It’s not like a captain of a ship tries to be a bad captain.)
Rider probably figures that if Shinji is evil, then she might as well have fun being evil too. So even when she and Shinji cause an uproar, she doesn’t think twice about it, “There isn’t gonna be anything left of us in the end anyways.”
As a Servant she does want to grant all her Master’s wishes, but she has a dry all-business attitude when it comes to the effort she puts in, and she refuses to use her powers which are above the level of her Master.

She strongly believes in having fun in life, and as such the end of her life will certainly be flamboyant.
She isn’t strongly attached to living… or perhaps I should say life. Neither is she attached to some deeper pride or meaning behind being a human. She thinks of death, which eventually knocks on everyone’s door, as good, and she loves it.
She is an adventurer by nature, and a pioneer.
Pushing forward, conquering, plundering, and prospering.
The true value of such things is drawn out only when overcoming ever greater troubles or people who are stronger than you are.
As far as Rider is concerned, the protagonist who had just woken up is a target which completely fails to motivate her desire for battle.

Fate/EXTRA Material: Encyclopedia of Fate/EXTRA