The Servant of the Sword
A beautiful and visibly unimposing girl around Shirou's age that nonetheless has the strength of a Dragon.
Her delicate-looking body is wrapped in imposing armour, and her sword is invisible, making her impervious to many attacks and allowing her to mask her attacks.
The Strongest Servant
... In theory.
Shirou's lack of apitude as a Mage and the botched nature of her summoning has weakend Saber and left her unable to enter spirit form, taking her from practically invincible to always at risk.
Unlike Shirou, who doesn't want to see anyone hurt, Saber is deadset on her goals regardless of human cost for those she is not sworn to protect. While she has some sense of chivalry, enough to give her real name to an opponent who does her, she will not hesitate to kill a defenceless teenage girl just because she is an enemy.
Combine that with her practial and cold personality and her lack of interest in anything outside of her goal, Saber can sometimes feel more like a fighting machine than an actual person.
She won't even tell Shirou her name, just her "role" as Saber.
But still, she is a person, and can't be older than 16.
What sort of life could lead someone so young to such a personality? And what goal is she unflinchingly working towards? What will she wish on the Grail?