Saber is Emiya Shirou's servant, an agile and powerful warrior. Saber is loyal, independent, and reserved; she appears cold, but is actually suppressing her emotions to focus on her goals. Her class is considered the "Most Outstanding", with excellent ratings in all categories. Because her master cannot effectively provide her with Mana, she minimizes her activity to preserve what she has. Saber is bewildered by Shirou's "protective" tendencies, and believes his erratic and reckless behavior jeopardizes her chances of winning the Holy Grail War.
Saber's true identity is Arturia Pendragon; her creation as a character was inspired by the legends of King Arthur. InFate/stay-night, the legendary King Arthur has been reimagined as a young woman rather than a man, and deeply burdened with the responsibility of upholding her ideals as what constitutes a rightful king.
Arturia is the daughter of the British King Uther Pendragon and Igraine, a former Duchess of Cornwall. At her nativity, Uther realizes his subjects will never accept a female as a legitimate ruler, and decides to not publicly announce Arturia's birth or gender. She is entrusted by Merlin to a loyal knight, Sir Ector, who raises her as a surrogate son. When Arturia is fifteen, King Uther dies, and with no known eligible heir to the throne, Britain enters a period of turmoil following the growing threat of invasion by the Saxons. Merlin soon approaches her, explaining that the British people will recognize her as a destined ruler if she withdraws Caliburn, a ceremonial sword embedded in a large slab of stone. However, pulling this sword is symbolic of accepting the hardships of a monarch, and Arturia will be responsible for preserving the welfare of her people. Without hesitation, and despite her gender, she draws Caliburn and shoulders Britain's mantle of leadership.
Arturia rules Britain from her stronghold in Camelot, and earns the reputation of a just, yet distant king. Under the guidance of Merlin and with the aid of her Knights of the Round Table, she guides Britain into an era of prosperity and tranquillity. Caliburn is destroyed, but Arturia soon acquires her holy sword, Excalibur, and Avalon, Excalibur's blessed sheath, from Vivian. While Avalon is in her possession, Arturia never ages and is immortal in battle.
Throughout her reign, Arturia is plagued by feelings of guilt and inferiority; she sacrifices her emotions for the good of Britain, yet many of her subjects become critical of her lack of humanity. Excalibur's scabbard is stolen while she repels an assault along her country's borders; when Arturia returns inland, she discovers Britain is being torn asunder by civil unrest. Despite her valiant efforts to placate the dissent, Arturia is mortally wounded by a traitorous knight, a homunculus born of her blood named Mordred, during the Battle of Camlann. Her dying body is escorted to a holy isle by Morgan le Fay and Sir Bedivere. Arturia orders a grieving Bedivere to dispose of Excalibur by throwing it back to Vivian; in her absence, she reflects on her personal failures, regretting her life as king. Before her last breath, she appeals to the world; in exchange for services as a Heroic Spirit, she asks to be given an opportunity to relive her life, where someone more suitable and effective would lead Britain in her stead.
Arturia is limited to the class of Saber, unlike many heroes that can be summoned into different classes, such as Hercules, who can be any class except Caster, and Cu Chulainn, who mentions that he could have been summoned as a Berserker instead of a Lancer.
In addition to the three Noble Phantasms she has access to at the beginning to Fate/stay night and Avalon, which she has in Fate/Zero and some routes of Fate/stay night, Caliburn is presumably her Noble Phantasm as well. However, she has no access to it in most of the game: as she is still alive, she is in the exact state as she was when she made her pact with the World, and at this point, Caliburn had been broken and lost. If "King Arthur" were to become a full Heroic Spirit, "he" might probably possess Caliburn as a Noble Phantasm. On a similar note, the Avalon we see is the actual original sheath which was recovered by the Einzbern family and given to Kiritsugu, and not one of the Noble Phantasms that came together with the summoning of Saber. She doesn't have a copy because, just like Caliburn, it was already stolen and lost.
Saber destroyed the Holy Grail in the fourth Holy Grail War because Emiya Kiritsugu used a command seal (Reiju) in order to force her to act against her will. However, she only managed to destroy the Grail's physical form. Her only wish is to redo her life so that she would never become king. In the Fate scenario, Shirou convinces Saber that she should accept her life for what it is rather than wishing to change her life. Saber only survives in Unlimited Blade Works' good ending. In Fate and Unlimited Blade Works, she used Excalibur to destroy the Holy Grail and its contents once and for all.
In the Heaven's Feel scenario, Saber is consumed by darkness by Angra Mainyu and turns into Sakura's Servant, Saber Alter. Saber Alter is later defeated and killed by Shirou.
In Fate/hollow ataraxia, Saber continues to protect Shirou, though it seems their relationship has turned more towards Saber having the major control over the relationship with Shirou. She kills Archer when he attacks Shirou in one loop. In another loop, Saber is killed by Bazett's Fragarach when she tried to use Excalibur on Bazett. In the final loop of Fate/Hollow Ataraxia, Saber aided Shirou/Avenger in reaching the Grail by fending off the mysterious monsters (which were in fact leftover parts of Avenger wishing to continue the loop) with other Heroic Spirits.
Saber made a pact with the World as she was dying; she would be allowed to try and obtain the Holy Grail to grant her wish. In exchange, if she succeeds, she will finish her life and become a Counter Guardian after her death, a sub-category of a Heroic Spirit at the service of the World. Because of this pact, she is in fact still alive and "frozen" in her dying state when she travels through time to obtain the Grail, and is summoned as a Servant in the Grail War.
After each Holy Grail War, she goes back into her dying body, unless she wins or gives up, ready for the next Holy Grail War.
Since Saber is not dead, she cannot dematerialize herself. This is also the reason she is able to retain her memories of the events of Fate/Zero, something a Servant should not be able to do.