CAT6

"They're a private military corporation called CAT6. As most of you know, CAT6 is an Alliance nickname for dishonorable discharges. They have many criminal records, histories of steroid abuse, and other charming features."

- Liara explaining CAT6

CAT6 is a private military organization introduced in the Citadel DLC of Mass Effect 3. Their name is derived from the term "Category 6", which is used for soldiers dishonorably discharged from the Systems Alliance. The mercenaries are confirmed to have multiple criminal records and histories of steroid abuse.

History
The CAT6 mercenary group 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 group. As Shepard gives chase after the Clone, the mercenaries 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 escapes and manages to sneak aboard the Normandy, The Clone instructs the mercs to slow down the Commander while it heads down to the armory for a final confrontation.

As Shepard and the Clone battle in the shuttle bay, Brooks and the mercenaries 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. None of the mercenaries who fought against Shepard survive; save for the one piloting the Normandy, who is subdued by security drones directed by EDI.

As Shepard and the Clone battle in the shuttle bay, Brooks and the mercenaries 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. None of the mercenaries who fought against Shepard survive; save for the one piloting the Normandy, who is subdued by security drones directed by EDI.