Known to be some of the strongest characters in the world of One Piece, the Admirals are incredibly powerful combatants who serve the Navy. Introduced first during the Long Ring Long Land arc with Aokiji, the Admirals were shown to be a menace the powers of which could even rival the strongest of pirates. With time, more and more of their ranks were revealed to the fans, with admiral Kizaru making his debut during the Sabaody Archipelago arc where he utterly destroyed the 11 Supernovas with minimal effort. Finally the strongest of the lot, Admiral Akainu was revealed to the fans during the Paramount War.
The Admirals are known to be the World Government's strongest military asset and, as such, their ranks must always include some of the strongest characters in the world. After the Paramount War, however, there were two major vacancies left behind by Aokiji, who left the Navy, and Akainu, who was promoted to the position of Fleet Admiral. Consequently, two new Admirals were chosen through a World Military Draft and these were Fujitora and Green Bull. While fans got to see a lot of Fujitora during the Dressrosa arc, Green Bull remained a mystery until the very end of the Wano Country arc. Now, however, fans finally know quite a bit about him and his powers.
Green Bull's Devil Fruit Power
Green Bull's Devil Fruit powers were alluded to during the Reverie arc for the very first time. In a conversation with Fujitora, Green Bull mentioned that he hadn't eaten in 3 years. This statement was rather bizarre as, even for the superhumans who live in the world of One Piece, eating is really important. However, if one relies on sunlight for food, this statement wouldn't make sense. Fans were quick to realize that Green Bull could have a plant-based devil fruit power.
In One Piece's Wano Country arc, Green Bull made his way to the land of Wano after the fall of Big Mom and Kaido, and fans caught a glimpse of his powers for the very first time. By sprouting a giant flower from his back, Green Bull was seen flying over the skies of Wano in an attempt to infiltrate the country.
As such, it is confirmed that he is a Paramecia-type plant-based Devil Fruit user. While the exact name of the Devil Fruit isn't known just yet, its powers have been seen already. Based on his recent showing, Green Bull's Devil Fruit allows him to produce plants and manipulate them according to his will. These plants offer him the power to strangle his opponents in a way that even some of the strongest characters, such as King and Queen of the Beasts Pirates, can't break free.
What's more, Green Bull can produce root-like extensions from his body using which he can pierce his opponent and absorb all their moisture, leaving them a husk of their former selves. This was seen in his confrontation with King and Queen in Udon, where he easily took down the Commanders, although it must be noted that both were already injured at this point. Green Bull also seems to have the ability to affect the terrain entirely. Udon, which was a barren wasteland, was turned into a lush green forest within a short period of time thanks to his powers. Everywhere Green Bull walks gets immediately covered in some sort of vegetation. While this could be a standard Devil Fruit power of his, it could also mean that his fruit is Awakened as it is affecting his surroundings.
Green Bull's Haki and Swordsmanship
Along with this overpowered devil fruit that he possesses, Green Bull is also most certainly a very adept user of Haki. Haki is a power that dwells within everyone in the One Piece world, however, only a few can utilize it.
That said, it was already established that every Marine with the rank of Vice-Admiral or higher has the ability to use Haki. As such Green Bull is, at the very least, a very skilled user of both Armament and Observation Haki. The ability to use Ryuo would also not be too foreign to him, as all the Admirals pre-time skip were seen using it. Interestingly, Green Bull also carries a sword at his waist, as seen in One Piece chapter 1053. While he didn't use his sword at all in his confrontation with the pirates at Udon, he appears to be a rather skilled fighter with the sword.
This is hinted at by his very sword itself, which appears to be a black blade – a very rare occurrence in the One Piece world. So far, only two other characters have blackened their swords, one being Dracule Mihawk and the other Shimotsuki Ryuma.
While it remains to be seen whether he truly possesses a black blade or not, he would be skilled with its usage, either way, being an Admiral of the Navy. If his blade is indeed black, then that would be another indication of how refined his Armament Haki is. With the power to take down two of the Yonko Commanders and confidence great enough to challenge even a Yonko such as Luffy himself in battle, it is no surprise that Doflamingo referred to Green Bull as a monster.