In this theory i would discuss about the past of Kaido and his relationship with Edward Newgate, also known as Whitebeard.
I want this to be a quick, short thread and I want to give credits to Boris Anime of Youtube because I heard this theory from him, I would expand on his theory and add some more details.
I would like to say this outright: I believe Kaido is the original first mate and right hand man of Whitebeard.
First off, It is interestingly weird that Marco the Phoenix, his current first mate, seems to be a young adult just by looking at him, unless his Phoenix fruit grants him youth.
But, even his whole crew is at least half his age, excluding Blackbeard, which I believe is older than he looks.
Even half of Whitebeard’s crew aren’t even strong enough to stand up to Shanks, while its not a definite hint that they are greenhorns, Shanks also never seem to know Marco which is weird since Marco is Whitebeard’s first mate.
Unless he is new on Whitebeard’s Pirates.
Also, the next thing to observe is that upon Ace’s recruitment aboard Whitebeard’s ship, Ace was offered for the 2nd Division Commander position, and it was noted that it had been open for sometime.
So by assumption, its either the 2nd commander died or left, or Marco was the former 2nd Division Commander and he got promoted to being the first mate.
Eitherway, it meant that there’s a missing person in the Commanders of Whitebeard, and I believe that was Kaido.
If we base it on the 4 Guardians of Japanese Mythology, we would determine that Kaido is based of Seiryuu the Blue Dragon and Whitebeard is based of Byakko the White Tiger.
While it doesnt mean much, Seiryuu guards the East and Byakko means West, so there had to be some connection between these two.
And if that’s not enough, I would say Whitebeard and Kaido are Yin and Yang, which when together are a good pair, but also contrast each other really bad.
And what does Yin and Yang represents as well? Dragon and Tiger, Seiryuu and Byakko: I believe this is the reference for Kaido and Whitebeard.
I would assume that since Kaido and Whitebeard had differing opinions, Kaido took half of Whitebeard’s crew with him and moved on to became the Kaido Pirates.
And that’s when Marco and other commanders came in for Whitebeard, reason why Kaido also seems to hold a grudge against Whitebeard and it manifests when he tried to intercept Whitebeard during the “War of the Best” at Marineford.
*Theory by beck26