Saber

セイバー, King of Knights

Height: 154 cm
Weight: 42 kg
Three Sizes: B73/W53/H76
Blood Type: O
Hair Color:
Blonde
Eye Color:
Green
In Fate/stay night:
She is Shirou's Servant, an agile but powerful warrior. Loyal, independent, and reserved, Saber acts coldly but is actually suppressing her emotions to focus on her goals. Her class is considered the "Most Outstanding," with excellent ratings in all categories. Since her Master cannot effectively provide her with Mana, she minimizes her activity to preserve what she has. Saber is frustrated by Shirou's "protective" tendencies, believing his erratic and reckless behavior will jeopardize her chances of winning the Holy Grail War.
In Fate/Zero:

Saber is summoned by Kiritsugu Emiya to participate in the Fourth Holy Grail War on behalf of Jubstacheit von Einzbern and the Einzbern family. Kiritsugu partners Saber with Irisviel von Einzbern, to act as Saber's Master in the open while he acts from behind to win with his own methods. On the final day of the Holy Grail war, Saber found the Holy Grail but she also meets Gilgamesh who was waiting for her. He proposes that she becomes his wife while she lays defeated in front of him. Saber refuses as the Holy Grail is just in front of her.Saber destroyed the Holy Grail in the fourth Holy Grail War because Kiritsugu used multiple command seals in order to force her to act against her will. However, she only managed to destroy the Grail's physical form. She was also caught in her own Excalibur blast, sending her back to the battlefield of Camlann. She was the sixth and final servant to perish in the fourth Holy Grail War.

(Source: Type Moon Wiki & Wikipedia)

Her true identity is that of King Arthur aka. Arthur Pendragon (Uther's daughter who hid her gender to be accepted as King). She has joined the Holy Grail War in order to grant her wish of redoing the selection of King due to her belief that she failed to keep her country from chaos. She wields the legendary holy sword Excalibur (cloaked in a veil of wind to hide its popular identity and thus its owner's) but no longer carries its lost sheath Avalon.