Aramaki

Ryokugyu (緑牛) is the alias of an admiral of the Marines. He/she was briefly mentioned by Doflamingo.

Appearance
As he/she has not appeared in the storyline yet, his/her current appearance is unknown.

Abilities and Powers
Not much is known about his/her abilities, but as a Marine Admiral, Ryokugyu must have at least above-average leadership and co-ordination skills, and has power over thousands of lower ranked Marines. As an Admiral, Ryokugyu has the authority to enact a Buster Call on any island he deems dangerous, and may pass this ability to a lower ranking World Government agent.

He and Issho were stated by Doflamingo to be "true beasts in terms of power".

Haki
It was stated that all Marines ranked vice admiral or higher possess Haki, so as a higher ranked officer as well as leader of the Marines, Ryokugyu can also use the ability.

Past
Not much is known about Ryokugyu's past, but sometime during the two years the Straw Hats were separated, he/she became an admiral.

Dressrosa Arc
Ryokugyu was briefly mentioned by Doflamingo whilst talking to Fujitora, calling him/her a "Beast" along with Fujitora.

Trivia

 * Green Bull has the same naming scheme (color + animal) as the four admirals before him.
 * Fujitora and Ryokugyu's animal aliases (tiger and bull respectively) are the representation of the Northeast Temple in Momotarō legend where the ogre that Momotarō must kill lives.
 * It's very likely that as the first original admirals were classified with the three primary colors (blue, yellow and red) this second set will be named after the secondary colors. This is in fact proved by Fujitora being "purple" (blue+red) and Ryokugyu being "green" (blue+yellow).