In Level 37 there is a "secret" switch that, when found and activated, gives the player and Proxy two additional allies that are SLC-56s. They are armed with Alien Shotguns and can also pick up a hidden CS-BNG.
There is a bonus level at the end of the campaign, where you play as the Android SLC-56. Your objective is to open all 5 doors, while evading Civil Security Ghosts. You are only armed with a Railgun v04 CS-OneSOneK.