The Mystery behind Kaido is Surrounding a lot ,some say Kaido is the Third mate of Roger: Scopper Gabang But here I have Theory revealing the real person behind Kaido Name.
Basically Lets starts the Theory with relation between Roger and WB. They used to fight a lot in past .
WB was the only man capable to fight on par with Roger ,same goes for WB pirates against Roger pirates Until now we have seen Two of Roger Pirates One being Rayleigh and Other being Crocus.
Point is while Roger used to fight WB , Who used to fight with Rayleigh ?
As Rayleigh was and even now powerful enough to be Threat to Marine.Marco seems Young/Weak Compare to Rayeligh .Something does not fit well
So here is a Twist, The one who used to fight with Rayleigh was the Previous Righthand Man/Former Firstmate of Whitebeard .
Reasons:
First Time Marco was introduced Shanks says, Oh! you’re the Captain of the First division implying that they really don’t know each other. Whitebeard knew Shanks since who was a cabin boy in Roger Crew so its unlikely that Shanks did not know anything about WB /his crew. Because in the past they used to clash a lot with each other,it suggest that Marco was not well known/famous in Roger Era .
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Garp being One of the Major Figure from Roger Era called Roger,Shiki, WB, Rayleigh(Right hand man of Roger) a legend so how come he says Marco as Brat .It means Garp didnot knew who was Marco,But it does make sense as WB’s Righthand man should be powerful enough to be recognized by Marine .Therefore In past WB righthand man was someone we do not know Yet .
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Age
Whitebeard, Roger,Crocus,Rayleigh are all around same age But Compare to them Marco seems too Young Now even if the age between the commanders differ there isn’t anybody as Old as Rayleigh ,even if Marco in Young could Match against Rayleigh it does not make sense as Marco was little/No threat to Marine at War .Meanwhile, on the other hand Old Rayleigh was strong enough to bring Blood of Admiral in short match .
Who was The Righthand Man/Firstmate of WB ? Where is he ?
I assume that the man who was Former Firstmate of WB is now One of the member of Yonkou Known as Hundred Beast of Kaido.
Reasons:
Prediction:
Kaido being able to predict Movement of WB While Marine who had greatest Intelligence Network also seems to had lost track of WB before the War.It shows to some extent Kaido predict the WB Movement ,where he is and what WB was planning, it fits well since Kaido was One of the closest Member of WB in past.
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Ephithet
Whitebeard The Individual known as Strongest Man in the World and Kaido being Called Strongest Creature seems Familiar just Like Roger known as Pirate King and Rayleigh known as Dark King.
They both have Similar ephithet.
Kaido Motives
Kaido attempt to attack WB was somehow unexpected since WB was known as Strongest Man in the world so other Yonkou seems to avoid him But Kaido tried to attack him….Why…..???
The Main motive behind Kaido assault was he knew the WB was Sick,powerless to some Extent. Thats why he was ready to ambush WB+His allies .Kaido had hunch that he could win against him since WB was way beyond his prime and Kaido had every Information of WB his Current power,Status,health etc.
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Shanks was able to stopped Kaido when he tried to attacked WB ,it was unexpected situation.
How was Shanks able to know that Kaido would attack Whitebeard,it does not make sense as Yonkou activity are not known by fellow Yonkou. Shanks to some extent knew that Kaido would surely try to attack WB .
Somehow Shanks had guessed it,because in past Kaido had bear Grudge against WB .
Shanks was able to negotiate with Kaido hints that they donot have just mere Yonkou relation but something Much More deep.
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The 2nd division commander seat was vacant until Ace came . Meaning Marco (Current 1st division commander) was the 2nd division commander back then ,Jozu third and so on. WB First-mate Kaido flee from the ship after his dispute with WB making the 1st division seat vacant so to fill that seat Marco even though he was young became 1st division commander leaving 2nd Seat vacant. It would explain why there was no 2nd division commander/Vacant 2nd seat when Ace arrived.
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Why Kaido defect from WB pirates ?
As we know Kaido is ruthless,power-hungry and ambitious. When he was with WB he thought that WB would become PK ,in contrast to him , WB had no interest in becoming PK,knowing that Kaido lost his respect for WB and set out to become next PK on his own .
Trying to surpass WB of what he could not achieve that is Becoming Pirate King
Conclusion- Kaido was Former Firstmate of Whitebeard and was Rival to Dark King Rayleigh.

