Heroic Spirit of the sword. A type of Servant that requires above average abilities in every basic parameter except for MGI.
Also, one of the main heroines of the story. A maiden knight that formed a contract with Emiya Shirou and fought to the very end of the Holy Grail War.
Her personality is righteous and thoroughly serious, and any sort of femininity she might have had has long since been usurped by her pride as a knight.
…However, if you strip away that veneer, a girlish aspect befitting her age will reveal itself.
She holds grudges when she's sulking, and gets so sucked into competitions that she totally forgets about her surroundings. She gets overly excited about delicious food, and frets about making mistakes in front of the person she likes.
Honestly, she's a hard to please heroine, umu.
Her true identity is King Arthur.
She disguised herself as a man to become king and brought peace to her homeland, but afterward lost her life during a civil war that tore her country apart from within.
The King Arthur in Fate/stay night is treated as a fusion of myth and historical fact. Her name before becoming king was Artoria.
Living in the castle while hiding her gender was an endless string of stressful situations. The matter of producing an heir was particularly troublesome, but it seems that she managed to get by without any problems thanks to Merlin's spell. In the story, when she said that “she knows how to please a man,” she was speaking from actual experience.
She stopped aging the moment she pulled the sword of selection, so physically she's about a year younger than Shirou.
Also, King Arthur is the red dragon that protects Britain.
Due to an arrangement her father, King Uther Pendragon, made with Merlin to produce an heir bearing the essence of a dragon, Artoria was born as a human infused with a dragon's magical energy.
Consequently, she has no need for magic circuits. Her every breath, the very circulation of her blood, it all naturally generates magical energy. It wouldn't be wrong to call her very being a “magic reactor,” on a completely different level from magi.
———Fate/stay night was originally her story.
At its conception, the main themes revolved around King Arthur and the Holy Grail, and that fact alone has never changed.
A maiden knight who galloped through the Holy Grail War.
I want to believe that the dream she saw at the very end was one worthy of her life.

Fate/side material: Fate Encyclopedia

A symbol-like existence. Without her, Fate wouldn't be Fate. However, in Fate/Zero her kingly shell is still intact, and she plays the role of a hero instead of a heroine.
Over the course of Zero she experienced neglect play and tentacle play, was stalked, slandered, leered at, and preached at by drunks, and to top it all off the scoundrel who ambushed her at the very end turned out to be a former ally. It's no exaggeration to say that the entirety of Zero is built on her suffering. Still, all of that was for the sake of enjoying Shirou's delicious cooking ten years down the line. Or at least, that's the excuse I gave to the Saber Maid figure that silently watched over my furious writing every day. If it had been the plain clothes version, I definitely wouldn't have been able to finish the story. Even just seeing one in a store window was enough to make me drop to my knees, ashamed of my own ugliness, and yell to the heavens, “What kind of man am I, bullying such a pure and beautiful young girl like this!?”
By the way, about Saber's suit. When I first imagined it, for some reason I just naturally assumed it would be a tuxedo. But after the art actually came in, I ended up settling on the current not-too-mannish traditional suit. Thinking about it now, making her patrol around a rustic suburban city like Fuyuki in full party dress would have been extraordinarily embarrassing for her, so it was probably the right call.

Fate/Zero material: Fate/Zero Encylopedia