Kiryu was created from the remains of the original Godzilla, reanimated by EDF.
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 landed in Tateyama 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 to dredge up the skeleton of the first Godzilla from Tokyo Bay and use it as a skeletal framework for constructing a MechaGodzilla? In short, fight atomic fire with atomic fire (in its case masers).
The plan was executed, with cloned DNA from the bones inserted into the quasi-organic computer controls, 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 it was Godzilla were reawakened by the modern Godzilla's own roars. Kiryu broke free of its pilot, Akane Yashiro, and proceeded to trash Tokyo, ignoring attempts at manual override. Godzilla left the city and headed back to the ocean, with Kiryu continuing its rampage 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 mecha's control again, Kiryu was sent out.
The second battle went well until Kiryu's receiver was damaged. Akane wanted to enter Kiryu and repair it herself despite orders against it. 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, a damaged Absolute Zero Cannon, and empty powercells.
In 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, appeared to warn Japan. They claimed that building Kiryu using the bones of the Godzilla that appeared in 1954 was a violation of the natural order of things, attracting the current Godzilla. The twins further claimed that Mothra would gladly take Kiryu's place in protecting Japan if they agreed to dismantle the mechanical beast, if they declined, Mothra would attack Japan. Since Kiryu had been built partially to defend against a second attack from Mothra herself, the Japanese declined.
But when Godzilla appeared and killed Kamoebas, Mothra voluntarily engaged him, and Prime Minister Hayato Igarishi was shamed into aiding her. Kiryu flew into battle again. Despite the team up, 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 larva, Kiryu was able to use his drill in his right arm to injure Godzilla's chest even more, eventually reaching and tearing up his insides. While Godzilla screamed in pain, Kiryu fired his Triple Hyper Maser and Maser Cannons at Godzilla, making Godzilla's roar trigger another flashback in Kiryu which was directly channeled to Yoshito. Godzilla was then wrapped up by the twin Mothra larva web. Unable to fight, Godzilla dropped to the ground, allowing Kiryu to win the battle. Kiryu then broke free of Akiba's control. Instead of finishing his battle, he merely subdued Godzilla, carrying him out to sea. After releasing Yoshito to safety, Kiryu plunged into the depths with his counterpart, never to be 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.
Kiryu is considerably sleeker than other versions of MechaGodzilla and bears a closer resemblance to Godzilla than the previous versions. While the 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's 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 oral maser cannons, 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 S.O.S., the flight pack was replaced by weapons pods where rockets were fired; leaving the attached thrusters on Kiryu's body 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 S.O.S., 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 White Herons.
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 fires a ball of energy that flash freezes its target, thereby causing it to disintegrate 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 S.O.S. The Triple Hyper Maser Cannon could be fired simultaneously with Kiryu's mouth cannon, which would cause Godzilla agonizing pain.