Monkey D. Luffy’s growth in Wano reaches a turning point during his imprisonment in Udon, where he begins mastering Advanced Armament Haki. This phase is crucial not just for his personal power progression, but for the larger conflict against Kaido. Until this point, Luffy’s attacks, even with Gear Fourth, were not strong enough to significantly damage the Yonko. That limitation forces him to rethink his approach to combat and haki.
In Udon Prison, Luffy meets Hyogoro, a former Yakuza boss who introduces him to a more advanced application of Armament Haki known as Ryuo. This technique allows the user to project haki outward, bypassing external defenses and damaging opponents internally. As explained in Eiichiro Oda’s One Piece manga during the Wano arc, this form of haki is essential for penetrating Kaido’s nearly indestructible body. Luffy’s training in Udon therefore becomes the foundation for his eventual ability to challenge one of the strongest characters in the series.
What Is Advanced Armament Haki (Ryuo)?
Advanced Armament Haki, referred to as Ryuo in Wano, is fundamentally different from the basic hardening Luffy used earlier. Instead of simply coating the body or attacks, Ryuo allows haki to flow outward like an invisible force. This enables the user to strike without direct contact or to send haki inside an opponent’s body, causing internal destruction.
Hyogoro explains that this technique has been used by Wano samurai for generations. Luffy initially struggles with the concept because he is used to compressing haki into his fists. Learning to let it flow freely requires both mental discipline and precise control. This distinction is critical because it transforms haki from a defensive coating into an offensive tool capable of bypassing even the toughest defenses.
The importance of this ability becomes clear when considering Kaido’s durability. Standard attacks bounce off him, but Ryuo creates a way to deal real damage. This makes Advanced Armament Haki one of the most important power upgrades Luffy gains before the Onigashima raid.
Luffy’s training in Udon is far from conventional. He is forced to fight while wearing seastone cuffs, which weaken his Devil Fruit powers and stamina. Despite this disadvantage, he uses the environment as an opportunity to focus entirely on haki control. This restriction actually accelerates his growth, forcing him to rely on technique rather than brute strength.
Hyogoro guides Luffy through practical demonstrations rather than theoretical lessons. One key moment involves removing explosive collars using Ryuo, showing how haki can be projected precisely without physical contact. Luffy repeatedly practices this technique, gradually improving his ability to emit haki outward.
The training also includes intense combat situations where Luffy must apply Ryuo under pressure. Fighting enemies in Udon helps him refine his timing and execution. By the end of this phase, he begins to grasp the fundamentals, even if he has not fully mastered the technique yet.
Why This Power-Up Is Crucial Against Kaido
Luffy’s previous defeat against Kaido highlights a clear problem: raw strength is not enough. Even his strongest attacks fail to penetrate Kaido’s defenses. Advanced Armament Haki directly addresses this weakness by allowing Luffy to attack from within.
This upgrade is essential during the Onigashima battle, where Luffy finally starts landing meaningful hits on Kaido. While it does not instantly guarantee victory, it levels the playing field significantly. It also sets the stage for further haki evolution, including Advanced Conqueror’s Haki.
More importantly, this training reflects Luffy’s adaptability as a fighter. Instead of relying on new forms alone, he develops a deeper understanding of haki mechanics. This shift in approach is what ultimately allows him to compete with top-tier characters in the New World.
Luffy’s Udon training is not just about gaining a new ability. It represents a major step in his journey toward becoming Pirate King. By learning Ryuo, he aligns himself with the traditions of Wano while also evolving beyond his previous limits.
Thematically, this moment reinforces one of One Piece’s core ideas: true strength comes from growth through hardship. Luffy’s willingness to learn, adapt, and push himself in extreme conditions is what sets him apart. His Advanced Armament Haki becomes a symbol of that evolution.
In the broader story, this training also bridges the gap between earlier arcs and the final saga. It prepares Luffy for increasingly powerful enemies and more complex battles. Without mastering Ryuo in Udon, his later achievements in Wano would not have been possible.
