The MS-11 Act Zaku is a high-performance, close combat MS designed to succeed the Zaku II and developed as part of the "Pezun Plan" near the end of the One Year War. Headquartered in Side 3 and with an arsenal built in the small asteroid Pezun, the "Pezun Plan" is a top-secret weapons development plan drew up by the upper-level of Zeon's military to create next generation mobile suits to help break the war's deadlock.

The Act Zaku used by Kycilia's Forces has a unique head crest, and was equipped with joint drive system that used the same field motor as Earth Federation Forces mobile suits. Sections of the suit used Zaku II's equipment, such as the backpack which was caused by delays in the development of new thrusters and fusion reactor. It also used some of the Zaku II's weaponry and was likely a test type[2]. Later given the model number "YMS-11", its original model number remains unknown.