Material-S is a Material, part of the system Unbreakable Dark materialized as a clone of Nanoha Takamachi during the Dark Fragment Incident.

Her name is first introduced in The Battle of Aces in English Material-S and in Japanese "Exterminator of Starlight" without specific ruby-kanas. The "S" in her name is believed to stand for "Stars" or "Starlight", which are both associated with Nanoha.

In The Gears of Destiny, it is revealed her rubied name is Stern the Destructor, which may be shortened as Stern. Stern is German for "Star."

Her device is Luciferion.

(Source: Nanoha Wikia)