Ruler's True Name is Jeanne d'Arc, the Saint of Orleans. A Catholic saint born in Domrémy, France, and the heroine of France who liberated Orléans in the Hundred Years' War.
A Ruler Class Servant is the Standard-Bearer who Correctly Guides the Holy Grail War. Jeanne is a special Heroic Spirit summoned by the Holy Grail itself to act as the administrator of the Holy Grail War instead of the Church. Being summoned in the Ruler class also allows her to retain the memories of each repeating "game" unlike other Servants.
Jeanne d'Arc was a common farmer's daughter. She claims she had received the command of God to fight. She heard the Lord’s lament that the world changed straight into hell. The Lord wept into sorrow as no one could stop it and people were not even allowed to live simply, and were compelled to become either beasts or food. Conflict never ended, and blood continued to rain incessantly and soak the land. She received a revelation from the Lord, the voice contained no glory or victory, no obligation or sense of purpose but only the Lord laments. She caught his small, feeble murmurs that everyone else failed to hear.
Jeanne also has a minor cameo in the second season of Fate/Zero.