Ryuzo was a childhood friend of Jin Sakai. Though not a nobleman or samurai, he did have some standing growing up as he was friends with a clan leader and was able to invite other clan leaders to watch the duel between himself and Jin. While the two were earnest friends, Ryuzo became bitter after his loss to Jin in Lord Nagao's tournament, both for the lost opportunity to formally raise his status in life, and for the ferocity at which Jin attacked him during the duel itself. Having lost his chance in becoming a Samurai, he joined the Straw Hats and grew up as a ronin.
Ryuzo leads the Straw hat ronin group after their leader's death. The Mongol invasion causes the group to down by half as they suffer from massive losses during the battle and starvation, leading to some members abandoning the group. When he hunts for food in the forest nearby Kishi Grasslands, he reunites with Jin. Jin asks him to help him rescuing Lord Shimura from Castle Kaneda. Ryuzo agrees to join Jin if he helps the Straw Hats raid Fort Ohira for food. While they are unable to find food, they obtain a map of supply routes which contain food and battleplans.
Later on, Ryuzo asks Jin to help rescue his men who are caged near Fort Yatate. After freeing his men, Ryuzo discovers that the Mongols didn't hurt his men; rather, the Mongols had fed them well. Wanting to feed and protect his men, he betrays Jin as he invades Castle Kaneda with his allies and informs him that the Straw Hats now side with the Mongols. Ryuzo and Jin duel each other, with Jin emerging victorious. Despite his betrayal, Jin pleads with Ryuzo, begging him to help save the island of Tsushima. Badly injured and outmatched by his old friend, Ryuzo instead alerts nearby Mongols before escaping the Castle Kaneda.
Khotun Khan, displeased by the loss of Castle Kaneda, takes Ryuzo with him to take Castle Shimura. To atone for his failure, Ryuzo is forced to burn a innocent peasant alive to coerce the castle's defenders to open the gates and breaks down in guilt and tears when the castle does surrender, traumatized by his deed.
As Jin continues to liberate Mongol camps and disrupt Mongol activities throughout the island, Khotun grows impatient with Ryuzo and orders him to kill Jin. However, Ryuzo convinces him to give Jin a second chance, and lures Jin into an ambush so that the Khan can try to convince him to join the Mongols. The Khan fails to convince Jin to surrender, and after Khotun kills Jin's friend Taka for his defiance, Jin escapes, slaughtering Ryuzo's remaining men and swearing revenge on Ryuzo and Khotun.
After Jin infiltrates and poisons the Mongol army during the Siege of Castle Shimura, he enters the inner keep to find and kill Khotun, but encounters Ryuzo instead. When Jin demands to know where the Khan is, Ryuzo explains that the Khan had already left to campaign further north, and made Ryuzo stay behind to collect Jin's head. Ryuzo does not want to fight and asks Jin to lie to his people and tell them that Ryuzo was Jin's spy. Jin, unable to forgive Ryuzo for his betrayal and indirect involvement in Taka's death, demands him to surrender and answer for his crimes, but Ryuzo, knowing that Lord Shimura would execute him on the spot, refuses. The two former friends then engage in a final duel, with Jin besting Ryuzo. Wounded, Ryuzo is barely able to plead with Jin. Unsheathing his tanto, Jin bids farewell to his old friend, before stabbing him in the abdomen, killing Ryuzo and disfiguring and decreasing morale in the Straw Hats.
Ryuzo is someone who always wants to be a leader rather than a pawn, even if it was in unconventional ways. For instance, Jin is surprised that Ryuzo is the leader for the Straw Hats, but Ryuzo passes it off as the Straw Hats needing a leader. Ryuzo also cares deeply for the Straw Hats, and will do anything to provide for them regardless of how difficult, such as when he burned a peasant alive (albeit reluctantly) to obtain food from the Khan.
Ryuzo shows admiration and jealousy towards Jin, which stems back to their first duel at Lord Nagao's tournament. Ryuzo had invited many leaders to watch the duel, only for him to lose to Jin and consequently lose almost any chance of ever becoming samurai. Additionally, when Jin announces his plan to the Straw Hats when he and Ryuzo reach the Straw Hats' camp, Ryuzo pulls him aside and tells Jin that those are his men, not Jin's, and that any plans should've been discussed in private beforehand.
Though not discussed in great detail, Ryuzo was greatly embittered toward Jin after the duel during Lord Nagao's tournament; the aftermath of the tournament left him functionally humiliated, in addition to losing his chance to become a samurai. On a more personal note, Ryuzo noted that Jin had attacked him during the duel as if Ryuzo was Jin's mortal enemy, an act that likely caused Ryuzo to think Jin didn't value their friendship.
Despite this, Ryuzo is a realist, and is shown when the Khan orders Ryuzo to kill Jin; Ryuzo admits that Jin is the better swordsman and that he wouldn't be able to kill Jin in a duel, leading Ryuzo to suggest an alternative.
- Ryuzo's name could mean "Noble" and "Three.