Roronoa Zoro is the swordsman of the Straw Hat Pirates and the man considered to be the second in command of the crew. Zoro is an exceptionally powerful fighter, as is evident from his 1.111 billion berries bounty. Over the years, he has grown into quite a vicious fighter, capable of taking on even the strongest of characters in the entire series. Zoro possesses three swords at his disposal, which are his primary weapon.
Through these swords, he deploys powerful techniques, some of which can end his enemies in a single strike. While the fans are familiar with most of Zoro's techniques, there are a few that only a handful of people know that he possesses.
4 Ryuo
Ryuo is, simply put, Haki in the world of One Piece. While on the surface, this doesn't look like an ability that most fans won't know about, taking a deeper look into the grade of Ryuo Zoro uses certainly does provide one. Zoro can use the basic level of Armament Haki and fans already know that, having seen him use it on countless occasions.
However, after receiving Enma, the sword started drawing out his inner Haki very efficiently. Zoro's Ryuo was upgraded to a point where unleashing more of the sword could allow him to cut through Kaido's scales. This meant that, in a way, Zoro was using an advanced grade of Armament Haki. This was further confirmed during his fight against King, when he could deflect his sword using a blast of Armament Haki from a distance, which is what the Advanced Grade of Armament Haki, also known as Ryuo in Wano, is capable of doing.
3 Potential Life Return
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Life Return is an ability that fans were introduced to in the Enies Lobby arc. Some of the assassins of the Cipher Pol, such as Kumadori and Lucci, are very adept at using this technique. Simply put, using life return, a person can manipulate every single aspect of their body, and this applies to the movement of their hair, the shape of their body, even things such as thirst or hunger or a digestive process, among many other bodily functions.
All of them can be controlled by the user at will. Using this Life Return, Lucci has been seen manipulating his body shape on various occasions. Zoro, on the other hand, has done something similar with his Nigori Zake, where he buffs up his biceps to produce powerful blows. This is very similar to Life Return, however, he certainly does not have full control over this ability. In the future, perhaps, Oda will explain the basis of this power of his, however, it is a very intriguing ability that, perhaps, doesn't receive as much attention as it should.
2 No Sword Style
Another one of Zoro's powerful techniques is the No Sword Style. While Zoro primarily relies on his swords in combat, he can also fight without them, however, he's significantly weaker while doing so. Zoro is at his strongest when he has all three swords. With two swords, he is still very strong, however not at his strongest and, similarly, with one sword, is even bigger. When he doesn't have a sword, Zoro is at his weakest, however, he can still use powerful sword techniques, such as Tatsumaki.
This is an incredible ability and fans have seen it happen on quite a few occasions in the story. It isn't known how Zoro manages to pull this off, however, it just goes to show that even in the most desperate of situations, Zoro can find ways to fight back. Undoubtedly, he's always among the very best fighters in the Straw Hat Crew, even when he doesn't have all of his swords.
1 Foxfire Style
In Wano, Zoro ended up stealing a technique that belongs to Kin'emon, known as the Foxfire Style. The Foxfire Style is unique to Kin'emon and it allows him to cut through flames and even use flames in his attacks. He can light his swords on fire and slice through any flame effortlessly. Zoro stole this technique from him and used it to slice through Kaido's Borobreath on Onigashima's rooftop.
Later on, he also incorporated the Foxfire Style into his King of Hell Three Sword Style, and this ability allowed him to cut through King's Karyudon, along with other powerful abilities. Had Zoro not learned this power, he would certainly have lost against King, and this just goes to show that every single skill that Zoro possesses right now is invaluable to him. In the future, Zoro will pick up even more niche abilities from here and there and incorporate them into his sword style and fans are certainly very excited to see that happen.