Yuri is known as the Leader of the Purple Sky, which comes from the Tome of the Purple Sky. Mahô Shôjo Lyrical Nanoha A's Portable -THE GEARS OF DESTINY-: Born in the Ancient Belkan ages, Yuri was originally a human. However, due to a program written with the aim of overwriting the Book of Darkness, Yuri was turned into Unbreakable Dark, aka System "Unbreakable Dark" (shortened as U-D) until the separation by the end of The Gears of Destiny. Mahô Shôjo Lyrical Nanoha INNOCENT : Yuri Eberwein, along Stern, Levi and Dearche, is a student studying abroad in Uminari City. (Source: Nanoha Wiki)