Developed very late during the One Year War as part of the Principality of Zeon's "United Maintenance Plan ", which was created to upgrade and improve already existing mobile suit designs to produce more streamline and effective manufacturing lines by including similar parts and systems into several different mobile suit lines. Under the "United Maintenance Plan", the MS-06FZ Zaku II Kai was built off of the original but outdated MS-06F Zaku II mobile suit line. The Zaku II Kai was not the only mobile suit line to be upgraded as part of the Plan: the MS-09RII Rick Dom II and the MS-14Jg Gelgoog Jager lines were also created.

The MS-06FZ Zaku II Kai featured a slightly more powerful Minovsky fusion reactor and a redesigned and improved cockpit. The Zaku II Kai had two different "helmet" designs. The "A-Type" helmet, which was the more common design, was designed like the typical helmet of the Zaku II series. The "B-Type" helmet was modeled after the Stahlhelm helmet used by the German army in World War II. The Zaku II Kai's main improvement came from its greatly increased thruster acceleration and overall improved maneuverability. However, this came as a drawback to the Zaku II Kai. Because of its increased thruster output, the Kai used double the fuel that its F-Type brother used. This cut the Zaku II Kai's operational battle time in half from the typical Zaku II.It also used a titanium and ceramic composite armor, which proved more resistant to both, regular ballistics and and beam weapons, it also was ligther than the previous armor, leaving more space for fuel, thrusters and additional equipment

Finally, it’s weapons were also upgraded, using a more advanced 90mm machinegun, which although fired shells of coger caliber, these had a higher penetration and an improved firing rate. The machinegun itself was equipped with a grenade launcher, which although was a one-shot weapon, it proved to be highly effective. On the other hand, it's Heat Hawk remained the same used by other Zaku units. It also could equip as well some sturm fausts, which although they were one-shot weapons, that were capable of destroying a mobile suit in a single shot. Finally, it had an improved grenade rack which made access to the grenades easier.

Due to the MS-06FZ Zaku II Kai's late introduction into the One Year War, only a handful of the units were produced. Few, if any, of the mobile suits survived the war.