Luffy didn't fully realize his potential until he properly trained Haki in the One Piece series. Haki allows users to manifest their spiritual energy in various ways. It's entirely based on the user's own willpower. Ever since the two-year timeskip, One Piece has maintained a strong focus on this fighting system.
Naturally, there is no way that Luffy would survive the second half of the series without Haki. One Piece fans may be thinking about the exact starting point of his learning experience.
Note: This article reflects the writer's personal views.
It took a while for Luffy to properly learn any Haki in the One Piece series
Rayleigh proved himself to be an instrumental figure
The Straw Hats had already witnessed Haki long before the One Piece timeskip. Back in the Skypiea arc, Enel would display tremendous feats with Kenbunshoku Haki. With that said, the concept was known as "Mantra" back then.
It wasn't until Sabaody Archipelago when the Haki system began to really develop. Luffy first met Rayleigh back in the Human Auctioning House. The Straw Hat just knocked out a World Noble for hurting his friend Hatchan. Luffy's crew also fought several guards to free Camie from slavery.
Rayleigh made the process a lot easier for them when he knocked out their enemies with his Haoshoku Haki. He took a liking to Luffy right away, since he was nearly the spitting image of Roger himself. Rayleigh decided to help out the young captain in any way he could.
He would eventually train Luffy for over two years
After the events of the Marineford arc, Luffy went back to Amazon Lily to heal from his injuries. Not long after, he would also meet up with Rayleigh again. Roger's first mate offered to train Luffy for the next two years, since they both knew he wouldn't survive the New World in his current state.
Luffy spent the entire One Piece timeskip in Rusukaina. This is where Rayleigh taught him all three different types of Haki. Luffy was tasked with defeating over 500 strong animals on the island. However, this could only be achieved if he mastered Haki, which is exactly what happened by the end of his training.
It's only fitting that Rusukaina was right near Amazon Lily. Shortly after Sabaody Archipelago, Luffy was sent to Boa Hancock's island, where he learned the basic concepts of Haki. This is when the One Piece series really started to make this system more prominent.
Luffy had learned a lot from Rayleigh
Even after the One Piece timeskip, the story would regularly return to Luffy's training via flashbacks. Anytime the Straw Hat used a powerful new move, he would think about his training with Rayleigh.
For example, during the Whole Cake Island arc, Luffy had a grueling battle with Charlotte Katakuri that lasted several hours. The Sweet Commander could see directly into the future with Kenbunshoku Haki. Luffy knew that he could only win the fight if he learned that very same technique.
He remembered how Rayleigh forced him to sharpen his senses, just so he could grow as a fighter. This formal training proved to be very useful, since Luffy would've never been able to defeat Katakuri otherwise. Rayleigh is the best teacher he could possibly ask for.
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