This chapter is solely for the purpose of helping readers better understand the story of the period between King and Kaido. Most of it is factual, with a small amount of speculation.
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47 years ago, Sea Circle Calendar 1475, King was born in the Grand Line, originally named 'Arber', a remnant of the Lunarian (People of the Moon) tribe.
During his youth, between Sea Circle Calendar 1475 and 1488, King was captured by the World Government and used for endurance experiments. During this period, Kaido, who had not yet joined the Rocks Pirates, was captured multiple times by the Marines.
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Kaido was originally from the Vodka Kingdom. By the age of 10, he had become the strongest warrior in that country. Around 46 years ago, approximately Sea Circle Calendar 1476, he was betrayed by the King of Vodka Kingdom and captured by the Marines for the first time. However, he escaped relying on his powerful physique. His first bounty was 70,000,000 Berries.
At 1476 to 1478, having just left his kingdom, Kaido had no money for food. Whenever he was starving, he would be deliberately captured by the Marines, eat his fill, and then escape from prison.
At the Sea Circle Calendar 1478, when the 15-year-old Kaido was wreaking havoc on Hachinosu Island, Whitebeard conveyed an invitation from Rocks D. Xebec to Kaido. Subsequently, Kaido joined the Rocks Pirates as an apprentice pirate and met Charlotte Linlin.
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34 years ago, Youth Period, Sea Circle Calendar 1488. King was 13 this year, Kaido was 25.
Kaido, captured by the Marines again, heard the legends of King and the Lunarian tribe while preparing his escape. During the jailbreak, he rescued King, called him 'King', and invited him to join the Beasts Pirates.
During this period, King had not yet eaten the Pteranodon Devil Fruit, but as a pure-blooded Lunarian, he possessed black wings.
King was drawn to Kaido's immense power and believed he could change the world, thus joining the Beasts Pirates.
King always believed Kaido was the legendary 'Liberation Warrior' and stated he would assist Kaido in becoming the Pirate King.
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Inserting another historical story here, concerning Kaido's left hand.
King has always been active on the seas as Kaido's right hand, while Kaido's left hand was none other than the King of Lizards, Hanafuda.
He fought Kaido multiple times, eventually forming an alliance due to a common goal.
Years of combat fostered mutual respect between the two, leading to Hanafuda having immense trust in Kaido, so much so that he was willing to entrust his children to Kaido to raise.
His children are Ulti and Page One.
Hanafulza's hobby was collecting Ancient Zoan-type Devil Fruits.
King's Ancient Zoan Devil Fruit was most likely a gift from Hanafulza. Queen's and Jack's Devil Fruits were also probably thanks to Hanafulza.
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Another identity of Hanafulza is that he was one of the Seven Warlord of the defeated by Ace in Sea Circle Calendar 1517.
That's right!
The Warlord defeated by Ace was Kaido's left hand! After being defeated by Ace and stripped of his title by the World Government, Hanafulza was killed by pirates he had a grudge against while attempting to return wounded to the Beasts to find his children!
This history can then be extended to the later Marineford War!
When the Whitebeard Pirates went to Marineford to turn the tables, Kaido made a move but was intercepted by the God's Knight, Shamrock!
Although this history isn't explicitly confirmed in the manga, considering the grudge between Hanafulza and Ace, it's clear Kaido had some personal motive. (This part is speculation.)