We're going to make something called a bio-robot. It'll be a powerful weapon against Godzilla. A mechanical Godzilla to beat Godzilla.	„
	— Hayato Igarashi (Godzilla Against Mechagodzilla)

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The Millennium Mechagodzilla (メカゴジラ Mekagojira), more commonly known as Kiryu (機龍 Kiryū, lit. Machine Dragon), is a cybernetically-enhanced mecha created by Toho that first appeared in the 2002 Godzilla film, Godzilla Against Mechagodzilla.

Built around the skeletal remains of the Godzilla that attacked Tokyo in 1954, Kiryu was an advanced cyborg designed to defend Japan from the kaiju that frequently menace it, particularly the current Godzilla. Kiryu first saw action in 2003 when it was deployed to battle Godzilla when he surfaced at Hakkeijima. However, Godzilla's roar awakened the original Godzilla's restless spirit that was attached to Kiryu's skeleton, causing the machine to go berserk. Kiryu was recalled and his programming altered so this incident could never occur again, then was sent out against Godzilla a second time when he landed at Tokyo. Kiryu fought Godzilla to a draw, leaving him with a gruesome chest wound and itself losing an arm. Kiryu was rebuilt over the course of a year, and sent into combat with Godzilla again when he returned to Tokyo. Kiryu fought alongside Mothra and her twin larvae, and finally triumphed against his organic counterpart by reopening the wound on Godzilla's chest. The original Godzilla's spirit took control of Kiryu once again, and chose to sink itself in the Japan Trench with the immobilized Godzilla rather and deactivate rather than kill another member of its kind.

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Source: wikizilla.org