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A G-Stone-derived bladed weapon, capable of varying in sharpness based on the will of the user. Originally used by Cyborg Guy, it was stored in the rear of his GaoBrace and was his primary offensive weapon on the ground. It also served a fundamental role in the defeat of EI-01, when Guy used a fragment of it that had broken off during a fight with Pizza to locate EI-01's core. The Will Knife would not see use during the Primeval invasion, and it would be absorbed into Guy's body when he was turned back into a human at the end of the television series.
Another Will Knife would later be built into Evoluder Guy's ID Armor, in the same position and with the same properties. Guy was also shown to be able to power up this Will Knife with his Evoluder abilities. This knife would be destroyed with the destruction of his ID Armor during his first combat with Palparepa PLUS.
Finally, Genesic GaoGaiGar would mount a Will Knife, the only one to be used by a Super Mechanoid rather than a human. This (from which the Earth-made versions were likely derived) was formed from the head of GadgetGao (the tip segment of the tail), which would attach over Genesic GaoGaiGar's right hand and fold open to project the blade. Unlike the Earth-made counterparts, this Will Knife had a much more stylized design, and featured a curved tip. This version of the Will Knife often glowed orange while in use, being charged with the orange-colored Genesic Aura in order to cut through defenses more easily. This Will Knife is further noted in official sources to be able to cut through dimensional rifts (though this is not seen in FINAL or any current official GaoGaiGar work). Despite being seen self-destructing when faced with Palparepa Prajna's sword, the component apparently regenerated, as it is seen back on GadgetGao in later shots of the tail.