|“||They're a private military corporation called CAT6. As most of you know, CAT6 is an Alliance nickname for dishonorable discharges. Many have criminal records, histories of steroid abuse, and other charming features.||„|
|~ Liara explaining CAT6|
CAT6 is a private military corporation introduced in the Citadel Downloadable Content of Mass Effect 3.
Their name is derived from the term "Category 6", which is used for soldiers who were dishonorably discharged from the Systems Alliance. The mercenaries are confirmed to have extensive criminal records and histories of steroid abuse.
The mercenaries are known to name their assault squads after sharp objects. Examples include Rapier, Hatchet and Dagger squads, and an elite contingent among them is known as Razor squad.
CAT6 is hired by Shepard's clone and Maya Brooks to assassinate the real Shepard. They first make an attempt to kill Shepard at a popular sushi restaurant, but they fail and Shepard escapes. They later intercept the Commander and friends when they infiltrate the Archives; where the Clone reveals itself as the mastermind behind the assassination plot. As Shepard gives chase after the Clone, the mercs make numerous attempts to slow the Commander down.
After trapping Shepard and friends inside a vault and leaving them to die in the Archives, the Clone and Brooks hijack the Normandy SR-2 along with an army of CAT6 mercs. However, when Shepard and friends escape the Archives and manage to sneak aboard the Normandy, The Clone instructs the mercs to stall the Commander's team while it heads down to the armory for a final confrontation.
As Shepard and the Clone battle in the shuttle bay, Brooks and the mercs begin providing backup for the Clone. However, Shepard eventually manages to defeat both the mercs and the Clone; with the latter falling to its death from the Normandy. None of the mercs who fought against Shepard survive; save for the one piloting the Normandy, who is subdued by security drones directed by EDI.
Mercenaries and other Criminals