Material-L is a Material, part of the system Unbreakable Dark materialized as a clone of Fate Testarossa during the Dark Fragment Incident.

Her name is first introduced in The Battle of Aces in English Material-L and in Japanese "Assailant of Thunderblade" without specific ruby-kanas. The "L" in her name is believed to stand for "Lightning", which are both associated with Fate.

In The Gears of Destiny, it is revealed her rubied name is Levi the Slasher, which may be shortened as Levi. Levi is a pun on "Levin," an archaic word for "Lightning."

Her device is Vulnificus.

(Source: Nanoha Wikia)