Kojiro Ryuzuki is a Koga ninja operating during Japan's Sengoku period. He fights alongside Hayabusa, both driven by the murder of their parents by the immortal warrior Ginnai Doma roughly 1.5 years before the main conflict. This loss fuels his fight against Ginnai and Oda Nobunaga. He first appears with Hayabusa after a major battle, surveying the destruction and Nobunaga's expansion before finding the wounded Shishimaru Tenoh. Kojiro attends strategy meetings with other ninja leaders, witnessing Shishimaru's grief over fallen comrades. He confirms Shishimaru was the sole survivor of Ginnai's attack and defends him from criticism. Kojiro participates in multiple assassination attempts against Ginnai. In one mission, he and Hayabusa's team ambush Ginnai in his sleep, paralyzing him with darts and stabbing him repeatedly. When Ginnai survives and nearly kills Hayabusa, Kojiro distracts him with a thrown kunai. The team escapes through tunnels beneath Ginnai's compound after Nagatoji sacrifices himself. Following this, Kojiro collaborates with Shishimaru and Hayabusa to set a trap using Shishimaru as bait. During the final confrontation, Kojiro aids Shishimaru after Ginnai reveals his mechanized body and threatens a nuclear meltdown. When Ginnai's detachable head kills most remaining ninja, Kojiro survives and retrieves Ginnai's spare sword for the one-armed Shishimaru. Though injured delivering the weapon, he enables Shishimaru to finally destroy Ginnai. Manga portrayals expand his background and tactics. Kojiro impersonates Ginnai to assassinate Dosan Saito, ruler of Mino, and claims responsibility. When the real Ginnai reveals Dosan was hidden elsewhere, Kojiro is framed for the attempt. He then allies temporarily with Shishimaru to fight their way out of the castle, igniting gunpowder he planted earlier as part of his infiltration plan. Later, he helps Shishimaru lure Ginnai into a cliffside trap with explosives. After surviving Ginnai's nuclear meltdown threat via alien intervention, Kojiro considers surrender but is persuaded by Shishimaru to keep living.