Crystal is a rare mechanism that mysteriously generates on a persons body for no apparent reason; the chance that a Crystal generates is only one in ten thousand and, should a person grow a Crystal, the person becomes a Drive-Union. After the Wadatsumi Mass Vanish Incident, the amount of Unions increased tenfold. Union's who existed before the Wadatsumi Incident are collectively known as Irregular Types, who act differently to more recent Unions; there is little information on them. Unfortunately, those who become Unions will usually die, due to the mental instability causing them to commit suicide, or their Crystal reaching Phase 6 – one Union has survived infection, however, but this was because of unique circumstances surrounding his progression through the Phases.
The Crystal in a Drive-Union's body converts seithr into a unique ability that the Union can use, however, this ability comes at a price: the spirit of the Union becomes polluted and leads to extreme erratic behavior; adding onto this, the Crystal physically becomes a part of the Union, integrating itself with their own nervous system. Coupled with this ability, called a Drive, they will release a unique sound whenever they use it – something called a Discover Call, however, only other Unions can actually hear the noise. Should a Union devour too much seithr, they cause a Gainart explosion, although this is not natural, and is mainly attributed to Sechs Drive, which can transform people into Unions after activation. Another rare anomaly in the Union society are the Strange Reds – Unions who have repeatedly eaten another's Crystal, turning their own Crystal red and sending them into Phase 0, meaning that they no longer progress into the further stages as long they keep devouring more Crystals, effectively achieving a form of immortality. This process is known as Enhancing and also strengthens the Union's Drive.
The Mitsurugi Agency protects, manages and treats the Unions, originally, this was because of Sōichirō Unomaru's plot to harvest their Crystals and use them to fuel the T-system, but this was foiled at the end of XBlaze – Code: Embryo, leaving Mei Amanohokosaka, Tōya Kagari, and the last Es-N unit to hunt them down and bring them in for proper treatment. Kazuto Kotetsu is somehow involved with Unions.
Information suggests that children who develop a Crystal can't withstand its power.
During their short lifetime as a Drive-Union, Unions gradually undergo multiple Phases, common symptoms include a change in seithr consumption, their Crystal changing color, and a rapid decline in sane mentality. Of all the Phases, Phase 0 is the absolute rarest, with only three of them being known. Unions can jump into the next Phase as time passes on, or as they receive more damage in combat. The following are the known Phases.
- Phase 0 – Crystal color is crimson. All three of those in Phase 0 are collectively known as Strange Red.
- Phase 1 – Crystal color is clear. Beginning Phase for Unions.
- Phase 2 – Crystal color is a faint light blue.
- Phase 3 – Crystal color is blue. Mental stability begins to slip.
- Phase 4 – Crystal color is dark blue. The ability to tell friend and foe apart is lost.
- Phase 5 – Crystal color is a blueish black. Union will have become fully insane at this point.
- Phase 6 – Crystal color is black. The Union will dissolve into seithr moments in the Phase.
List of Drive-Unions
- Acht (deceased) – Drive: Veil Skin
- Akio Osafune (deceased) – Drive: Iron Shooter
- Akira Kamewari (cured) – Drive: Hekatonkheires
- Goro Joizumi – Drive: Screaming Jaw
- Mei Amanohokosaka (in her route only) – Drive: Blood Cain
- Ripper (deceased, Irregular Type) – Drive: Scissor Hands
- Sechs (deceased) – Drive: Deadly Sign
- Tōya Kagari (Irregular Type) – Drive: Unknown (controls blood)