The term "greatest" applies to many members of the Straw Hat crew, One Piece's series protagonists. With the Straw Hat captain, Monkey D. Luffy, aiming to find the fabled One Piece and claim the title of Pirate King, it's no wonder that his drive and dedication has attracted comrades that also strive to be the very best of the best. At Luffy's right hand is his first mate, the pirate hunter Roronoa Zoro, who seeks to become the greatest swordsman in the world and, throughout their adventures so far, it's safe to say that Zoro is well on his way.
Fans of One Piece know that Zoro is no traditional swordsman. While still incredibly strong wielding one or two swords, he is a master of Santoryu -- a style that has him wielding one sword in each hand and one in his mouth during a fight. As unorthodox as it may seem, Zoro's skill with a blade is largely unmatched and throughout the whole of One Piece, he has demonstrated time and time again how he makes the most of his three-blade arsenal.
Zoro's Santoryu Style
Despite his mastery of the style, Zoro initially preferred Nitoryu, the two sword style, when he began training at a young age. It was only after the unexpected death of Kuina, his childhood friend and greatest rival, that Zoro was bequeathed her sword, and thus began developing Santoryu -- the art of using three swords.
As of yet, Zoro is the only practitioner of the combat style -- no other character in the series has been shown attempting it. (Except for his "shadow" Jigoro.) This is noteworthy considering One Piece features a plethora of sword specialists. It's a testament to Zoro's skill that he developed the style on his own with no specific Santoryu teacher, and has not shared the secrets of the style with anyone.
Still, not every combat situation is ideal. Frequently, Zoro will default to using a smaller number of swords (or none at all) due to the specifics of the situation or the particular technique he intends to use. One of the biggest advantages of Zoro's multiple sword-usage is that he's able to use as many as he needs to to get any job done.
Zoro's Three Swords
While technically any sword will do, Zoro has collected blades more powerful than the standard. The idea behind this is simple: better-crafted swords allow Zoro to be stronger. Perhaps the most important sword to Zoro in his collection is Wado Ichimonji, a blade that is considered in the series to be a meito, or Legendary Sword. This sword was given to him by Kuina's father after her death, and one that Zoro has had in his possession since he set out to become the world's greatest swordsman. A majority of the other blades that Zoro has utilized are also considered meito: Sandai Kitetsu, Yubashiri, Shusui and Enma are all Legendary Swords that Zoro has wielded.
It's worth noting, though, that Zoro is incredibly versatile. He's capable of utilizing whatever he has available to suit his need. In an intense battle against Kamazo The Manslayer, for instance, Zoro was able to use a scythe that he was initially impaled with to pull off a powerful Santoryu technique. In his battle against CP9's Kaku, Zoro resorted to wielding Sogeking (the latter held one of his swords) because the two were handcuffed together. There's even a scene where Zoro uses three mops as weapons; truly the height of versatility in the One Piece world.
Zoro's Devastating Techniques
Using any of his three swords, Zoro can execute several different techniques. They range from basic physical sword strikes to moves that draw on spiritual power to alter Zoro's physical form. Whatever the situation calls for, Zoro usually has a move that he'll use to end the fight decisively. Oni Giri, for example, is one of the first techniques that Zoro reveals against Cabaji of Buggy's crew, and in its most basic form, it's simply a high speed and high powered slash.
Other techniques, like Zoro's Aura Ichibugin attack, function differently. By utilizing the strength of his soul -- likely through Haki or some similar means -- he can temporarily take the form of an Asura, a supernatural being from Buddhist mythology. This ability makes Zoro appear to have three heads, three pairs of arms and due to his Santoryu style, a total of nine swords. Fighting against Kaku, CP9's resident swordsman, Zoro used this technique to not only cause Kaku's strongest attack to disintegrate, but also to subsequently defeat him with ease.
Unsurprisingly, as Zoro grows in strength, so too do his techniques. During One Piece's time skip, Zoro is trained by his greatest opponent and the greatest swordsman in the world, Dracule Mihawk. After learning from the person he seeks to surpass, Zoro's skill exponentially increases. After the time skip, Zoro's Oni Giri attack has evolved into an even more powerful version, renamed Rengoku Oni Giri. This attack is much stronger, much faster and able to decimate opponents like Hyouzou and Kamazo -- and in a particularly well-animated display in the case of the latter.
Zoro is dangerous enough with one sword, but with three, he is virtually unstoppable. Dedicated to his captain, his crew and his dream, it's safe to say that since the beginning of the series, Zoro has established himself as one of the most powerful anime swordsmen of all time. Nothing could make such a character more iconic than an unorthodox yet highly effective three sword style.