Things You'll Need
Instructions
Attain the rank of lieutenant commander.
Go to the "Main Menu."
Select "Matchmakers."
Select "The Armory."
Select "Chest."
Scroll down and buy the "Collar/Breacher" chest armor. This is the chest armor Kat wears in the campaign mode. It costs 8,000 game credits, and unlocks Kat's prosthetic arm for purchase.
Scroll to the bottom of the chest armor list and find Kat's prosthetic arm listed as "Collar/Breacher [R]." You will need 75,000 game credits to buy it.