She is a young girl who serves under Luvia as a magical girl. Her origin is unknown, except that she has a brother who looks similar to Shirou Emiya, and she takes on the Edelfelt name after being taken in by Luvia. She lives in Luvia's mansion and acts as her maid to earn her keep. She uses Ruby's sister Kaleidostick, Sapphire, in order to transform, and she initially has a much better understanding of her abilities than Illya. She takes her role very seriously and treats Illya as a bothersome rival, though they eventually become close friends. She later gains the ability to use the Saber class card to embody the Heroic Spirit after seeing Illya perform it.
At a later part of the story, Rin Tohsaka informs everyone, including Bazett, that there was a 8th class card. Miyu and Illya are forced to compete against Bazett to see who will retrieve it first. They are overwhelmed by the Heroic spirit (Gilgamesh) that dwells within the mirror world after the Gilgamesh takes out his Noble Phantasm Ea. After they had returned, Gilgamesh followed them through to the real world, having destroyed the mirror world with Ea.
It was later revealed that Miyu is not a human from this world, but is from another reality. Gilgamesh further revealed that she was a complete Holy Grail and was only born for the Holy Grail War. After IIlya combined both kaleidosticks and defeated Gligamesh, Miyu was captured by two intruders from the other reality and brought back to her world.