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, along with U-D, were written a long time ago in the Ancient Belkan era; they were originally intended to take possession of the powers of the Book of Darkness, but were sealed by the Book (probably the Defense Program) in return.
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.
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).