Material-D is a Material, part of the system Unbreakable Dark materialized as a clone of Hayate Yagami during the Dark Fragment Incident.

Her name is first introduced in The Battle of Aces in English Material-D and in Japanese "Lord Ruler of Darkness" without specific ruby-kanas. The "D" in her name is believed to stand for "Darkness," which is associated with Hayate and the Book of Darkness.

In The Gears of Destiny, it is revealed her rubied name is "Lord" Dearche, which may be shortened as Dearche. It is most likely a misspelling of "Lord" Dearche. She is properly spelled as "Lord Dearche" in Magical Girl Lyrical Nanoha INNOCENT (video game).

Her device is Yersinia Kreuz.

(Source: Nanoha Wikia)