The Millennium Mechagodzilla first featured in Godzilla Against Mechagodzilla (Godzilla X MechaGodzilla in Japan), nicknamed Kiryu, was also a purely terrestrial creature.
Although the original Godzilla had been killed in 1954 by the Oxygen Destroyer, Japan still found itself under attack from other monsters such as Mothra and Gaira over the decades. When a second Godzilla appeared in 1999, this was the last straw. The Anti-Megalosaurus Force (AMF) division of the Japanese Xenomorph Self Defense Force (JXSDF) proposed a shocking plan. What better way to defeat Godzilla than this: dredge up the skeleton of the first Godzilla from Tokyo Bay and use it as a skeleton/framework for constructing a Mechagodzilla? Cloned DNA from the bones was also inserted into the quasi-organic control computers, intended to take over subconscious motor control functions for the crew. By 2003, the metal monster was ready. Kiryu was unleashed, but when the cyborg faced off against Godzilla, something horrific happened: the cyborg's own memories of when he was Godzilla were reawakened by the modern Godzilla's own roars. Kiryu broke free of its pilot, Akane Yashiro, and proceeded to trash the city (manual overrides being ignored) while Godzilla left the city and headed back to the ocean. Kiryu continued its rampage for a couple of hours until its fuel ran out.
The Japanese public was not amused. Hastily recalled, Kiryu received quite a few weapon upgrades in addition to new computer control software routines. When Godzilla attacked again a few days later, both branches of the JSDF tried stopping him without the steel dragon's help, but it was useless. With Akane at the mechas controls again, Kiryu was sent out. The second battle went well until Kiryu's receiver was damaged. Akane wanted to enter Kiryu and affect repairs personally, despite being ordered not to. She then piloted the cyborg directly from its skull, carrying Godzilla out into the bay once more. Akane tried to destroy Godzilla with Kiryu's ultimate weapon, the "Absolute Zero Cannon", but the fight ended in a draw. Godzilla went back to the sea (though not before suffering a horribly shredded chest) and Kiryu went back to the body shop after suffering a destroyed right arm, damaged Absolute Zero system, and empty powercells. Godzilla: Tokyo S.O.S., taking place one year later, saw Akane sent to America for further flight training. Taking her place in case Godzilla appeared was Akiba, but Kiryu engineer Yoshito Chujo is the human focus of the film.
Mothra's twin fairies, the shobijin, appear to warn Japan. They claim that building kiryu using the real bones of the Godzilla that appeared in 1954 was a violation of the natural order of things, sure to keep attracting the current Godzilla. The twins also claimed that Mothra would gladly take Kiryu's place in protecting Japan if they agreed to dismantle the mechanical beast. Since Kiryu had been built partially to defend against a second attack from Mothra herself, the Japanese declined at first.
But when Godzilla did appear and killed Kamoebas, Mothra voluntarily engaged him, and Prime Minister Hayato Igarishi was shamed into aiding her. Kiryu flew into battle again. Despite this, the current adult Mothra was killed and Kiryu's transmitter damaged once again. Just like Akane before him, Yoshito entered Kiryu for repairs, but remained inside the mech during the battle after Godzilla's beam had accidentally struck both of the hatch doors damaging and jamming it.
Teaming up with the newly-hatched twin Mothra larvae, Kiryu was able to use his drill in his right arm and injured Godzilla's chest even more, eventually reaching, and tearing up his insides. While Godzilla screamed in pain, Kiryu fired his Triple Hyper Maser at Godzilla, making Godzilla roar to show Kiryu his flash back to Yoshito. Godzilla was then wrapped up by the twin Mothra larvae web. Unable to fight, Godzilla dropped to the ground, making Kiryu win the battle. Kiryu broke free of Akiba's control. Instead of finishing his battle, he merely subdued him, carrying Godzilla out to sea. After releasing Yoshito to safety, Kiryu plunged into the depths with his counterpart and was never seen again. This marked the first time that Mechagodzilla was able to defeat Godzilla decisively by the end of a Godzilla film without Godzilla coming back to destroy him.
Mechagodzilla was intended to return in the next film, Godzilla: Final Wars. But with so many monsters being revived for the film, each with new looks, the budget costs were enormous. So too was time needed to squeeze all these kaiju into one film. Toho Studios finally nixed Mechagodzilla completely from the movie.
Kiryu is considerably sleeker than other versions of Mechagodzilla, and bears a closer resemblance to Godzilla than the previous versions. While Showa and Heisei Mechagodzilla are constructed of fictional alloys, it is never specified what Kiryu's armor is made out of. It is the first Mechagodzilla to include organic parts, built from the skeleton of the original Godzilla and using Godzilla DNA in its computer system. Additionally, unlike previous Mechagodzillas, Kiryu's tail was able to move which allowed Kiryu to use it as a weapon. Kiryu is a well balanced machine, and the most agile Mechagodzilla to date, moving with a fluid, almost organic grace. Its armament includes an oral maser cannon, dual laser cannons on each arm, and a flight pack that contains two forward firing rocket launchers and four batteries along its back and sides for launching guided missiles. Upon emptying its payload, the flight pack can detach from Kiryu's body and launched to push the enemy a safe distance back before being remotely detonated. In Tokyo SOS, the flight pack was modified so that only the weapons pods were fired; leaving the thrusters attached and allowing Kiryu to maintain its aerial capabilities. For close combat, a short blade can be extended from Kiryu's right wrist which was not only able to penetrate Godzilla's nearly impervious hide, but was also able to discharge a crippling electrical surge into his opponent's body. In Tokyo SOS, Kiryu's arsenal was upgraded to allow its right hand to collapse into a revolving drill. Like the previous Mechagodzillas, Kiryu was also able to fly; although its limited energy reserves required the machine to be airlifted to the battlefield by two carrier craft. Kiryu's most devastating weapon is the Absolute Zero Cannon, (a weapon also utilized by the Gotengo.) Stored in its chest behind a trio of folding panels, the Absolute Zero Cannon fired a ball of energy that flash froze its target, thereby causing it to disintergrate under the weight of its own mass. After being heavily damaged in Godzilla Against Mechagodzilla, the Absolute Zero Cannon was deemed too expensive to repair and was replaced by a Triple Hyper Maser Cannon in Tokyo SOS.