Developed very late during the One Year War as part of the "United Maintenance Plan", the MS-06FZ Zaku II Kai ("Kai" is Japanese Kanji character "改". In English means "Modified".) was an improved and redesigned version of the mainstay MS-06F Zaku II. The United Maintenance Plan was an effort by the Zeon military to simplify the production of their different types of mobile suits by standardizing parts and systems where possible.
The MS-06FZ Zaku II Kai featured greater acceleration and a redesigned and improved cockpit. It also had the option of two different "helmet" designs. The "A-Type" helmet, which was the more common design, was similar to the typical helmet of the Zaku II series. The "B-Type" helmet was modelled 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 also meant that due to increased fuel consumption the unit's operational time was shorter: about 50% of that of the original Zaku II. It also used a titanium and ceramic composite armor rather than the super high-tensile steel armor of the original Zaku, which proved more resistant to both regular ballistics and beam weapons. In addition, it was lighter than the previous armor, leaving more space for fuel, thrusters and additional equipment.
Finally, its weapons were upgraded. The Zaku Kai featured a new 90mm machine gun, the MMP-80. Although the MMP-80 fired shells of smaller caliber than the standard 120mm MMP-78 weapon used by standard Zakus, it had a higher penetration and an improved firing rate. The machine gun itself was also equipped with a grenade launcher, which despite being a one-shot weapon, proved to be highly effective. Additionally, the Zaku Kai had a new grenade rack on its right hip in which three cylindrical grenades could be stored for convenient access. Its Heat Hawk remained mostly the same as that used by other Zaku units. The Zaku Kai also could equip weapons used by other Zeon mobile suits, such as sturm fausts, which were a large-scale version of the Panzerfaust anti-tank weapon capable of destroying a mobile suit in a single shot.
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 of these mobile suits survived the war.
The most famous MS-06FZ Zaku II Kai appears in Mobile Suit Gundam 0080: War in the Pocket, which is piloted by Bernard Wiseman. He effectively uses his machine to critically damage the considerably superior RX-78NT-1 Gundam "Alex", albeit at the cost of his own life. The Zaku II Kai with the B-type helm was also the main unit of the Capitol Defense Battalion stationed in Zum City later in the war, while another unit was also piloted by Liberio Lincke of Zeon's regiment the Marchosias.
Years later, in U.C. 0096, one unit kept by the Zeon remnant forces participated in the attack on Torrington Base, where it was destroyed along with most of the attacking forces. This unit was equipped with the 280mm Bazooka.