First the Admirals before the timeskip.
Lets start with Aokiji, who we meet the first time at Long-Long Island. His name translates to Blue Pheasant and he has the ice-ice fruit.
The next admiral we meet is Kizaru, who we meet at Shabondy Archipelago. His name translates to Yellow Monkey and he has the light-light fruit.
The last admiral we meet pre-timeskip is Akainu, who we meet at the Marineford war. His name translates to Red Dog and he has the magma-magma fruit.
Now i believe the names have an extra meaning. Lets start again with Aokiji (Blue Pheasant).
Blue is a link to his power, the ice ice fruit, but it can also be a link to the blue sea (pirates). Aokiji was the only admiral pre-timeskip who fought with the strawhats because they were pirates (as main reason). It is hidden abit by the fact he wanted to deal with Nico Robin and later Spandam using his golden den-den-mushi to trigger the buster call.
The Nico Robin factor was for Aokiji something from the past. He had tried to let her escape in Ohara but she was spotted and got the bounty on her head at an early age. After twenty years on the run he thought she just wanted to die so he decided to put everything about Ohara to an end. The whole enies lobby thing was mainly CP9 and not Aokiji. Later he’s on Garp’s ship when Garp attacks the Strawhats when they leave Water Seven and he helps confirm that they escaped.
Now next is Kizaru (Yellow Monkey).
Yellow can be linked to the heavens (Celestial Dragons). When Luffy punched a Celestial Dragon it was Kizaru who volunteered to go deal with him and the other pirates at the human shop.
This was his main reason to go after them. When he meets Luffy again in the Marineford war he tells him that the Celestial Dragons have been clamoring for Luffy’s arrest.
Last is Akainu (Red Dog). Red can be linked to the Red Line (Revolutionaries). Squardo reveals after he stabbed Whitebeard that Akainu is a member of the rebel faction.
I believe this is the faction of the Marines that deal with the Revolutionaries. This is further hinted at him (Akainu) knowing about the relation between Dragon and Luffy before its announced in the war.
This is also his main reason for going after Luffy.
Some small notes about the animal parts.
– Pheasants are groundliving birds like for example chickens. This could link at Aokiji traveling by bike.
– Monkeys live in groups. This could link to how Kizaru works together with his nephew: Sentomaru.
– Dogs are very loyal. This could link to how Akainu places Justice above all. He’s loyal to the values the Marines stand for.
Now the admirals after the timeskip.
Now after the timeskip Akainu became fleet admiral, Aokiji left and 2 new admirals were drafted Fujitora (Purple Tiger) and Ryokugyu (Green Bull). We meet Fujitora (Purple Tiger) for the first time in Dressrosa.
From the way he is dressed we could speculate that he is from Wano Kuni. This gives more meaning to what Doflamingo said about the ‘World Military Draft’.
Brook explained that Wano Kuni isn’t affiliated with the world government and that their samurai swordsmen are so strong that not even the Marines go near there.
So it is abit strange to see an Admiral come from there. Fujitora (Purple Tiger) explained that he wants to abolish the Royal Warlod system which is most likely his reason for joining the Marines in the first place. The World Military Draft gave him the opportunity to not only join the Marines but also to become an Admiral. This gives him influence in the Marines and also the Reverie with which he can strive to his goal (abolishment of the Royal Warlord system).
Now if we apply the previous format to his name (Purple Tiger) then Purple can link to countries not affiliated with the World Goverment. Also Tigers are solitary but social creatures. This means that they hunt alone but can sleep in the same location as others from their species (Well males usually avoid each other but a male can have an overlapping territory with a female). This could link to how Fujitora does his own thing, even if that means disobeying orders.
Its been quite a long theory already but now we get to the part of Ryokugyu (Green Bull). We dont know wether Ryokugyu (Green Bull) is male or female so for this part i will adress him/her as he/him. Lets use the same format again (seperating the names) and start with the ‘Green’ part. I’ve seen a couple of theories that link his power to either trees (nature), radio-activity or time manipulation. But when i think of the colour green i think of healing (mainly because off all the hours i spent playing Pokemon when i was a kid). Because of this i believe his function before becoming an Admiral was a doctor, probably a Marine one. We know that the Marines have a Medical Centre.
If we take it that this is the case we can also speculate about his home country which would be most likely Drum Kingdom. Drum Kingdom was formerly renowned for its doctors and medical knowledge but when Wapol became king he made that law which only allowed the 20 doctors in the palace to stay.
All the other doctors were exiled from the country. There are some hints that could link Green Bull to Drum Kingdom such as Dalton having the cow-cow fruit model Bison
and also Dr.Kureha has been shown wearing a shirt with a bull on it.
If we continue with the format then next is the ‘Bull’ part. The most common known thing about bulls is that they go mental when they see the colour red. This could link to Ryokugyu (Green Bull) having a berserker ability where he looses control over himself and devestates everything close by.
I want to link this to Chopper and his Monster point. Chopper studied under Dr.Kureha and invented the rumble balls. When he took three of these he lost control over his devil fruit and went on a rampage abit like a berserker. This could prove an important hint. How exactly did Chopper invent those Rumble Balls?
He could have made them from scratch ofcourse but if we take it that Green Bull came from Drum Kingdom then he could have also used previous research that already existed and made the Rumble Balls from that research. If for example Green Bull was afraid of his own power and the fact he looses his mind whenever it activates then he could have tried researching a way to stop this berserk ability.
With the new admirals (Kizaru, Fujitora and Ryokugyu) we no longer have the logia theme which the old admirals had. Fujitora is a paramecia (since Blackbeard has the gravity logia) so its likely that Ryokugyu is a zoan. I believe he researched the Zoan fruit to explore the posibilities of stopping the berserk effect and left the research behind when he was exiled from Drum Kingdom (gave the research to Dr.Kureha most likely).
Later Chopper found that research and made the Rumble Balls. After his exile Ryokugyu, still afraid of his power, joined the Marines hoping that if he could not be cured they atleast could keep him in check. With the ‘World Military Draft’ Akainu forced him to become an Admiral because even if he looses control with his ability he is still insanely powerful.
Now i have saved the best for last and that is his devil fruit. If Chopper used his research for the rumble balls then it is likely they have a similar kind of Zoan devil fruit and that is the human-human fruit. I believe Ryokugyu (Green Bull) has the human-human fruit model: Monster/Berserker/Hulk.
With it he transforms into a monstrosity with insane physical abilities but he also looses control of his mind (like the real hulk). As an Admiral i believe he orders all his troops as far away from him before he activates his devil fruit (this would give some cool panels). The drawback from this might be that he can’t use Haki while in berserker mode which would make him useless against logia but still insanely effective against paramecia and zoans.
– Admiral names could hide hints to their affiliation and powers.
– Green Bull was a doctor from Drum Kingdom and later for the Marines.
– He has the human-human fruit model: Monster/Beserker/Hulk (one of these most likely)
– His fruit brings him into a berserker rage where he looses his mind, afraid of this ability he wanted to cure himself so did alot of research on the human-human fruit (later gave this research to Dr.Kureha upon his exile).
– Chopper used this research to make the Rumble Balls.