Rider's True Name is Kundry (クンドリー, Kundorī?), a woman cursed to live forever because she laughed at Christ's pain when He was crucified. She was involved in Percival's search for the Holy Spear of Longinus, which was part of his quest for the Holy Grail.
Kundry's immortality takes the form of her constantly reincarnating with her memories intact. Once, she had been a witch; once, the consort of King Herod II. It was even said that she was once a Valkyrie. It was her fate to serve men of strength in every life, only to be used as a tool and cast aside – and that fate would never end, until she was at last united with her true love, the Holy Knight of the Swan, and granted the salvation of death.
She is described as a beauty wearing a nun's habit and veil, with long black hair and Mediterranean skin. Her clothes were torn apart in her fight with Erice Utsumi to the point it could be called "scandalous", but she looked like a virtuous woman of the cloth before then.
Rider is described as a mad queen, and shows obvious signs of being unhinged. Rider will never tell a lie, but nor will she ever serve the side of good. She falls madly in love with the man she decides is her lover, such as Hendrik van der Decken, and refuses to listen when others try to convince her otherwise.
Rider arrived in Akihabara pursuing Hendrik van der Decken, a Servant whom she had decided was her lover. Erice Utsumi, a defender of Akihabara who hunts Servants that break the rules of the Holy Grail, recognized Rider as a threat because if let alone she would begin to feed on the life energy of innocent people in order to sustain herself. A fight between Erice and Rider destroyed Rider's mount.
Erice escorted Hendrik and his Master Ahasuerus out of Akihabara in an attempt to get Rider to leave as well. Rider had pursued them with her summoned gremlin familiars and fallen into despair when they left. Unwilling to leave the city herself, she pierced Erice with the Holy Spear and then vanished. As Rider's Saint Graph was falling apart from their fight, Erice wondered if she would make it very far before disappearing.
She can summon many Gremlins through unknown means that are believed to not be her own, and she had a mount but Erice Utsumi destroyed it. She is capable of using Projection magecraft to create the Holy Spear Longinus.
She is described as being cursed with immortality, but in a way that she would wander from world to world by reincarnating while keeping her memories until she would find her one true love. Because of this unique sort of immortality, Kundry as a Servant can remember individual summonings unlike how most Servants forget each time they are summoned.