The original Mechagodzilla was created as a weapon of destruction by the Simians.
It first appeared in a pseudo-flesh outer covering, masquerading as the real Godzilla during attacks against Japan in Godzilla vs. Mechagodzilla. Curiously, while the Simians gave Mechagodzilla a laser beam in its mouth to mimic Godzilla's atomic breath, they didn't bother replicating Godzilla's unique roar. Godzilla's ally Anguirus wasn't fooled by the impostor, but in the resulting fight Mechagodzilla broke Anguirus' jaw and sent him fleeing underground. Although the battle went badly for Anguirus, it tipped humanity off to the charade because while Godzilla and Anguirus had initially been enemies in 1955 in the second Godzilla film, they had been firm allies ever since, and the two were known to come to one another's aid in combat against other monsters. Anguirus attacking 'Godzilla' was seen as a complete shock. Anguirus had also exposed a piece of Mechagodzilla's true mechanical nature by ripping off a piece of the disguise the machine was covered in, though most humans did not seem to notice it.
The Simians rebuilt their dreadnought for another try in Terror of Mechagodzilla one year later. Having learned the value of teamwork firsthand, the Simians called in an old debt to pair Mechagodzilla with the aquatic dinosaur Titanosaurus that had been discovered by a Dr. Mafune.
This time there were some modifications made, mainly turning the mecha into a true cyborg by giving it living human brain cells. This was accomplished by integrating its control circuitry into the body of Dr. Mafune's daughter Katsura, as well as a variety of other cybernetic enhancements. Also Mechagodzilla's main control system was moved down into its neck so it could function unimpaired if Godzilla again attempted to decapitate it. Godzilla's perseverance combined with the timely self-sacrifice of Mechagodzilla's operator (Katsura killed herself) brought the machine down for good. The King of the Monsters buried Mechagodzilla's shattered form deep underground to prevent another repair job.
The original Mechagodzilla is the only one to be referred to by numerics within the movies themselves. When it is rebuilt in its second appearance, the "MG" emblazoned on its arm has a "2" added to it. It is still usually referred to as simply "Mechagodzilla" by the characters.
This version of Mechagodzilla was rated Number 15 of the 50 Best Movie Robots by The Times, beating other such legends as C-3PO from Star Wars, the T-1000 from Terminator 2: Judgment Day, and Optimus Prime from Transformers.
The Showa Mechagodzilla has armour constructed out of a mysterious and nearly indestructible alloy known as "Space Titanium." It is also equipped with a staggering amount of firepower.
Mechagodzilla is capable of firing a rainbow colored optical beam that was able to match Godzilla's atomic breath, known as the "Space Beam". Its head can rotate a full 360 degrees, allowing the robot to shoot down an opponent that might attempt to approach from behind. Mechagodzilla's head is also able to generate a cylindrical force-field around the robot's body by spinning around rapidly. When disguised as Godzilla, Mechagodzilla used a Flamethrower from its mouth to mimic the real Godzilla's atomic breath. After the disguise was destroyed, Mechagodzilla never used that weapon again.
The Showa Mechagodzilla is also equipped with powerful missiles in its fingers, knees, and even its toes. These missiles can explode on contact with a target or lodge in an enemy's flesh like darts. Located under a hatch in Mechagodzilla's chest is a weapon that fires a jagged orange energy beam with enough power to cut a mountain in half. Mechagodzilla 2 was able to increase the velocity of its finger missiles by rapidly rotating its hands and could also fire a ray from under its head when it was ripped off.
Mechagodzilla is shown to be capable of using its beam and missile weapons simultaneously and against multiple targets, at one time engaging Godzilla to the front and King Caesar to the rear.
The Heisei Mechagodzilla, from Godzilla vs. Mechagodzilla II, was created by the United Nations Godzilla Countermeasures Center using Futurian technology scavenged from the remains of Mecha-King Ghidorah. When Godzilla attacked the city of Kyoto, Mechagodzilla was deployed against him. The mechanical kaiju acquitted itself well, but was eventually defeated due to a voltage backsurge during their electrocution of Godzilla. One of the back-up pilots, Kazuma Aoki, suggested merging Mechagodzilla with the Garuda, a smaller airship that had been constructed by the UNGCC before Mecha-King Ghidorah's remains had been salvaged.
Kazuma, in fact the chief designer and main pilot of Garuda to start with, found some personal redemption as his idea is approved. The combined form of the two mechs, dubbed Super-Mechagodzilla, is unleashed against the King of the Monsters and Fire Rodan. In the heat of battle, Kazuma personally detaches Garuda and flies against Fire Rodan while the other Mechagodzilla crew continue against Godzilla. Kazuma is defeated, but manages to reattach the Garuda to Mechagodzilla to form Super-Mechagodzilla.
Super-Mechagodzilla proceeds to destroy Godzilla's secondary brain crippling Godzilla and then moves in to destroy Godzilla. Fire Rodan leaps to Godzilla's defense, but is fatally wounded by a blast from Super-Mechagodzilla's Megabuster Ray. Upon landing on Godzilla, Fire Rodan, with his last ounce of strength, places his beak on one of Godzilla's dorsal plates, releasing the radioactive energy within his body and healing Godzilla. As Fire Rodan dissolved, he filled the air with a superheated, radioactive dust, which melted Super-Mechgodzilla's diamond coating. As a result, Super-Mechagodzilla was rendered defenseless against the revived monster. Godzilla, temporarily supercharged with radiation, destroys the mech with his red spiral atomic breath as revenge for Rodan's death. Super-Mechagodzilla's crew escape uninjured.
The Heisei Mechagodzilla has an impressive and varied arsenal built only to destroy Godzilla and other monsters, which makes it a powerful adversary. Mechagodzilla is protected by an armor plating much like Garuda's NT-1 armor though Mechagodzilla's is made from a much stronger version of the alloy called NT-20, which is coated in a layer of artificial diamond that allows it to absorb energy from Godzilla's atomic breath and transfer it to its ultimate weapon, the plasma grenade, located on its abdomen. Mechagodzilla was also able to absorb energy from Fire Rodan's Uranium heat ray. This weapon could be rendered useless if the plasma pool used to channel the energy overheated, this diamond coating is the only real reason Mechagodzilla survived any of its encounters with Godzilla. Mechagodzilla also possesses laser cannons in its eyes and a mega buster beam in its mouth with destructive power equitable to Godzilla's atomic breath. It can fire tranquilizer missiles from its hips, paralysis missiles from the shoulders, and shock anchor cables from its wrists which deliver a powerful electrical surge into its opponent. After Godzilla used his conductive nuclear touch to reverse the surge and send a back surge through the cables and into Mechagodzilla, the shock cables were upgraded into a more powerful version called the G-Crusher which was used to destroy a secondary brain in Godzilla's lower spine.
Finally, Mechagodzilla can attach to the airship Garuda and becomes Super-Mechagodzilla. As Super-Mechagodzilla, it gains Garuda's powerful laser cannons as well as added thrust from Garuda's jets.
Mechagodzilla 2 is a well-known incarnation of Godzilla's mechanical nemesis because it was the incarnation that came closest to actually killing Godzilla - if not for Fire Rodan's sacrifice, Mechagodzilla 2 would have likely finished off the immobilized Godzilla.
The Millennium Mechagodzilla was featured in two films: Godzilla Against Mechagodzilla and Godzilla: Tokyo SOS.
In the Millennium version 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. Japan proposed a shocking plan: to dredge up the skeleton of the first Godzilla from Tokyo Bay and use it as a skeleton/framework for constructing a Mechagodzilla, also known as Kiryu or Mecha-G. Yet they were unaware that this bold move would only infuriate and provoke this new Godzilla.
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. Mechagodzilla was unleashed, but the cyborg's latent memories were reawakened by the new Godzilla's own roars the memories being Kiryu's attack on Tokyo in 1954 as the original Godzilla. Mechagodzilla broke free of its pilot, Akane Yashiro, and proceeded to trash the city (manual overrides being ignored) without the new Godzilla's help for a couple of hours until it ran out of power.
The Japanese public was not amused. Hastily recalled, Mechagodzilla received quite a few weapon upgrades in addition to new computer control software routines. When the new Godzilla attacked again a few days later, both branches of the JSDF tried stopping him without the steel monster's help, but it was useless. With Akane at the ROV controls again, Kiryu was sent out.
Mothra's twin fairies, the Shobijin, appear to warn Japan. They claim that building Mechagodzilla using the real bones was a violation of the natural order of things, sure to keep attracting Godzilla. The twins also claimed that Mothra would gladly take Mechagodzilla's place in protecting Japan if they agreed to dismantle the mechanical beast. Since Mechagodzilla 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 (but at the same time, reluctantly) engaged him, and Prime Minister Hayato Igarishi was shamed into aiding her. Mechagodzilla flew into battle again. Despite this, the current adult Mothra was killed and Mechagodzilla's transmitter damaged once again. Just like Akane before him, Yoshito entered Mechagodzilla for repairs, but remained inside the mech during the battle after Godzilla's atomic breath had accidentally struck both of the hatch doors damaging and jamming it.
Teaming up with the newly-hatched twin Mothra larvae, Mechagodzilla was able to use his drill in his right arm and injured Godzilla's chest even more, eventually reaching, and tearing up his insides, causing Godzilla to bleed. While Godzilla screamed in pain, Mechagodzilla 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. Mechagodzilla broke free of Akiba's control, but instead of finishing his battle, he merely subdued him and carried Godzilla out to sea. After releasing Yoshito to safety, Kiryu plunged into the depths with his counterpart and was never seen again. This is the second and final time Mechagodzilla succeeded in subduing Godzilla.
While The Showa era Mechagodzilla has been referred to as a cyborg, it was not until Terror of Mechagodzilla that the rebuilt Mechagodzilla had any interface with organic tissue, which came in the form of Katsura Mifune, who became the base control unit for the machine, and as such the Showa era version was a cyborg in only the loosest sense as the control unit was a remote system implanted into a true cyborg. The Millennium era Mechagodzilla however, is a genuine cyborg as it contains organic components within itself. These components enabled the third Mechagodzilla to act independently of any form of control, and as a result was capable of making decisions and taking certain actions on its own, performing movements and actions not programmed by any of the control systems, usually at catastrophically inopportune moments.
This feature was never intended by its creators and was an unforeseen development, however it proves to be a pivotal ability in Tokyo S.O.S., as it makes the decision on its own to take the modern Godzilla out to sea rather than kill him. Also unlike the previous Mechagodzillas, this one proved to possess some form of self-awareness, as well as an understanding of human language, communicating only once with Yoshito, a human who had worked with and on the third Mechagodzilla extensively, directly in Tokyo S.O.S. and indicating that it had been aware of Yoshito by name, though exactly how long the cyborg had been aware of him is never made clear.
Millennium Mechagodzilla is considerably sleeker than other versions of Mechagodzilla, and bears a closer resemblance to Godzilla than the previous versions, as a result of being literally built on the bones of the first Godzilla. 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, this one's tail was able to move which allowed him to use it as a weapon.
Millennium Mechagodzilla 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 (detachable in "Tokyo S.O.S"), 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 his 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 Mechagodzilla to maintain its aerial capabilities. For close combat, a short blade can be extended from Mechagodzilla's dual laser cannons 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. Also in Tokyo S.O.S., Mechagodzilla's arsenal was upgraded to allow its right hand to collapse into a revolving drill. Like the previous Mechagodzillas, this one was also able to fly; although its limited energy reserves required the machine to be airlifted to the battlefield by two carrier craft.
Millennium Mechagodzilla'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 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 Hyper Maser Cannon in Tokyo S.O.S.