The Materials (マテリアル Materiaru) are programs of the "Eternal Ring" of the Unbreakable Dark (U-D) System in the portable continuity. They were initially one of the constituent programs along with U-D of the Tome of the Purple Sky, which was later added to the Tome of the Night Sky in an attempt to control the U-D. After Nanoha and Fate refuse to seal Reinforce and the Darkness of the Book of Darkness wakes, they are spawned by it as mages and given the powers and appearance of Nanoha, Fate and Hayate Yagami.
The nature of the Materials is similar to that of the Wolkenritter, and their existence can slightly be felt by Reinforce in the past. However, they were not materialized until the Defense Program was destroyed. Each of their materialized forms is a copy of one of the mages who defeated the Defense Program.
In The Battle of Aces, they are misidentified as three special "dark fragments" created by the Defense Program's regeneration. In The Gears of Destiny, it is revealed that they were actually materialized due to the unsealing (regeneration) of the system Unbreakable Dark. It further reveals that the Materials once together with the U-D used to make up the Tome of the Purple Sky. Which is where their driving need to find the U-D came from.
Within the game, the Materials are secret characters who combine the strengths of their bases with those of two others, making them extremely powerful (e.g. Material-S has Vita's skill in melee and Zafira's defense in addition to Nanoha's abilities).