One Piece's Wano Country arc saw Luffy and the Straw Hat Pirates finally take on their first big step on the grand stage by taking on one of the Four Emperors of the Sea, Kaido. Being a Yonko and, more importantly, the strongest enemy that Luffy had to face to this point, it was expected of him to establish himself as a significant character in the arc, just as all the biggest antagonists did in their arcs. The likes of Crocodile established himself as an incredible character as did Doflamingo, who is widely considered to be one of the best, if not the best, antagonists of the entire series.
As the Wano Country arc went on, Kaido started to firmly position himself among the very best antagonists of One Piece, and by the end of his fight against Luffy, he was already a fan favorite. For some, Kaido was easily the best antagonist that the story had delivered to this point already, however, for others, Kaido needed a bit more to make himself rank among the very best. Many fans expected Kaido's flashback to come up at some point as this would give them an insight into his way of life and his thinking as a pirate. Although fans had to wait quite a lot for it to actually happen, eventually, Kaido's flashback arrived in One Piece 1049, shortly before his defeat at the hands of Monkey D. Luffy.
Kaido's Harsh Childhood
Kaido's exact race and origins weren't revealed in his flashback. Oda introduced the then-young Oni as a 10-year-old strong enough to make an entire country, called the Vodka Kingdom, fear him. It was believed that, at this point, Kaido was already the strongest soldier that they had but he craved more. Using him in wars, the Vodka Kingdom was able to generate enough funds to pay their Heavenly Tribute and stay in the good books of the World Government. However, to gain a place in the Reverie, Kaido was sold to the Marines. Although they succeeded in capturing him, Kaido managed to escape from his shackles again and again. In fact, he got caught on purpose just so he could fight and eat, something that earned him the reputation of a madman. At the age of 15, Kaido went to Beehive Island, where Rocks D. Xebec had set camp and upon Whitebeard's invitation, he ended up joining the crew. However, shortly after, the Rocks crew fell apart.
By this time, Kaido had already established himself as an incredibly powerful pirate, making everyone who saw him fear him. As brought up countless times before, Kaido formed a close bond with Big Mom during this time and in all the crew, was the closest to her. The crew eventually headed to God Valley, where they attacked the Celestial Dragons and, more importantly, fought their biggest battle. Unfortunately for them, the Roger Pirates and the Marines joined forces to take them on and succeeded in defeating Captain Rocks. At the same time, Rocks Pirates fell apart as well, mostly due to infighting which they were already known for. It is on this day that Kaido ate his Mythical Zoan Uo Uo no Mi, Model: Seiryu.
Kaido's Rise To The Pinnacle Of Piracy
As stated before in the story, all the former members of the Rocks Pirates ended up getting much stronger and made a great name for themselves. At some point after the events of the God Valley Incident, Kaido was captured by the Marines again, perhaps, on his own whim. Before he escaped, however, he ended up freeing King alongside himself and recruited him as the very first member of his crew. 10 years after the God Valley Incident, Kaido had already made a huge name for himself. He came to be known as the Embodiment of Strength, meaning he was already an exceptionally powerful pirate by this time. On Higurashi's proposition, Kaido headed to Wano Country to get weapons and seize power once and for all.
Having witnessed war from the very beginning and fighting to survive, Kaido's mentality was shaped to believe that's the way of life. According to him, peace could never be attained without brutality and all humans were essentially animals. As such, survival of the fittest is what Kaido started to live by. This was evident from his countless actions over the course of the story and, at the same time, from the very name of his pirate crew. Kaido leads the Beasts Pirates because he believes that all humans are essentially beasts and that only by fighting can anyoneprove their worth. To gain more power, Kaido eventually moved to Wano Country by agreeing to Higurashi's proposition and manufacturing more weapons there. In the time that Kaido spent there, he established himself as a major power in the world by amassing a great number of weapons and trading them.
Kaido then went on to fight Oden in a battle that he won thanks to Higurashi's help and, interestingly, he acquired his goal of waiting for Joyboy here. According to Yamato, Oden wanted to open Wano Country for the arrival of Joyboy, and for Kaido, this became the ultimate reason for staying in Wano. Kaido wanted to be defeated by Joyboy and after waiting for long and building his strength up to the point where he became the strongest creature, he awaited the day when Joyboy would come to Wano and eventually beat him.
Kaido's wish was, eventually, granted to him in the Wano Country arc, when Monkey D. Luffy carried the weight of the whole country on his back and promised to create a world where none of his friends would ever starve. Luffy became Joyboy in the midst of defeating Kaido and finally gave the Yonko what his heart desired for so long. All that said, Kaido is a phenomenal antagonist who, just like Oden, awaited the arrival of Joyboy, just for different reasons. Although he's lost, he went on to establish himself as an incredible antagonist and will go down as one of One Piece's best so far.