Edelfelt, Miyu

She later gains the ability to use the Saber class card to embody the Heroic Spirit after seeing Illya perform it.

Miyu is the complete Holy Grail from the world in which the Class Cards originated. Her brother is that world's version of Shirou. Brought to the main world through unknown means, she was taken in by Luvia in her search to rival Ilya and Rin's partnership.

In 2wei, Miyu continues to fight as a Magical Girl, dealing with the mysterious 8th Class Card. After dealing with Illya's split personality, Kuro, Miyu and co. fight the Enforcer, Bazett. They defeated Bazett and cooperated with her to capture the 8th Card.

The 8th Card was revealed to come from another Holy Grail ritual, and it gains physical body, child Gilgamesh. Later it was revealed by Gilgamesh that she comes from a parallel world and she is a princess. After saved from black Gilgamesh by Illya, Miyu was captured by Angelica and Beatrice from another world.

In 3rei, Miyu was captured and held captive in Ainsworth's stronghold.

Later it was revealed her original name was Miyu Sakatsuki (朔月美遊). She was a "Child of God," a child from an occult religion that worships children below 10 years old as gods, and this granted her wish-granting power, not unlike the Holy Grail. Kiritsugu and Shirou found her in an incident that brought her hometown into enormous black fire, and witnessed Miyu dispel the fire. Kiritsugu took her, wishing to utilized her wish-granting power to save the world.

Kiritsugu asked Shirou to use Miyu to grant his wish of world salvation, but Shirou couldn't do it because Miyu was becoming more and more human everyday day. Kiritsugu died some years later, and Shirou continued to take care of Miyu. At this time, Shirou and Miyu became really close. Miyu then use her power to wish for her and Shirou to become real siblings.

(Source: Type-Moon Wiki)