Suō Yagyū is the final member of the Yagyū lineage, humanoid weapons engineered to fight the Shinobi – monstrous entities that arrived via an asteroid and ravaged the world. He is engineered with enhanced physical capabilities: superior concentration, speed, and strength, inherent traits of his bloodline. His distinct hereditary markers are gray or white hair and sharp blue eyes.

Found near death, he is rescued by Ame, former princess of the fallen nation Ise. Her act of saving him, coupled with her praise for his eyes – which she finds beautiful and human – sparks his unwavering loyalty and developing romantic feelings. He joins her quest to locate and activate the Holy Weapon Giga Road, humanity's potential defense against the Shinobi.

Throughout their mission, he grapples with his identity, seeing himself as a monster akin to the Shinobi due to his nature as a weapon. This internal conflict intensifies under societal persecution targeting his kind despite their protective role. His relationship with Ame deepens into a star-crossed romance, marked by mutual affection but culminating in separation. During the final confrontation, Ame sacrifices herself to stop the Shinobi and their leader, Tsukiyomi, leaving him the sole survivor; he is later shown grieving her loss.

Key relationships include Sanda, an older swordsman serving Ame who acknowledges his Yagyū traits, and Tsukiyomi, the antagonist manipulating Shinobi forces against them. As the last Yagyū, he stands as both a relic of humanity's engineered defenses and a symbol of its lingering hope against annihilation.