Green Bull is an admiral in the Marines. He attained his rank during the two-year timeskip, along with Fujitora, both filling the two admiral vacancies left by Aokiji and Akainu. His real name is currently unknown, “Green Bull” being his alias as an admiral.
We at this point know that Fujitora has the “gravity” devil fruit Zushi Zushi no Mi. Green Bull is called a “beast” by Doflamingo in terms of “power”. There is no mystery surrounding Fujitora about his ability, but we don’t know about Green Bull, who is explicitly mentioned by Doflamingo.
As an admiral, Green Bull, with the other admirals, is thought to constitute the World Government’s “Greatest Military Power” (i.e. the three strongest single combatants it can bring to bear), making him one of the world’s most powerful people.
Green Bull’s devil fruit hasn’t been officially announced yet, but it is apparent that he has a plant fruit. As others have said he has a green theme, the color theme of each admiral lines up with their fruit and he has not eaten physical food in years which could be explained by photosynthesis.
Obviously in chapter 1052, he displayed his power by sprouting a dandelion from his back and floating with it.
As we have seen, each admiral has their own distinct style of doing their jobs to the fullest extent, or justice as they call it. Akainu is absolute, Kizaru is unclear meaning usually the most entertaining option to him, Kuzan is lazy justice and lets the situation play out until he sees it necessary to intervene which is usually in a very moral way, and Fujitora who uses blind justice to judge a situation for how good or bad it is regardless of outside circumstances or norms.
I think, like his fruit Green Bull has a form of justice like a plant. He knows that the plants will come back eventually and overtake any kingdom that rises up but maybe he seeks to expedite the process. I think he has Restorative Justice, meaning he wants to restore a situation back to its natural state, good or bad. This is why I think him being on Wano is important. He will restore the lush forests of Wano and bring it back to its former glory.
I see him like Swamp Thing from DC, sort of ambivalent to the affairs of the world as long as the natural order is restored at the end. Perhaps he wants to restore the world back to the way it was in Joyboy’s time and he’s the Forest God spoken of in Skypiea.
Theory by scaptastic (https://www.reddit.com/user/scaptastic)