Little is known about the past of the four emperors. Only Big Mom, aka Charlotte Linlin, was in part better known than the others due to the flashback narrated during the Whole Cake Island saga showing the past of ONE PIECE's Strongest Woman. With the Onigashima Saganow it's Kaido's turn.
The Emperor has been part of the Pirate Rocks in the past, but not much else was known about him. However, Chapter 1049 of ONE PIECE starts over and reveals Kaido's story. Kaido was born in the realm of vodka, a territory of the Grand Line under the yoke of the Celestial Dragons, to whom they constantly pay tribute. With the kingdom in shambles and only the fittest surviving: Kaido has made it a mantra, excelling in violence at just ten years old. The boy is then captured by the king's soldiers and handed over to the navy to avoid having to pay another tribute to the sky dragons.
However, Kaido manages to escape the naval ship and makes a name for himself wandering around, earning a 70 million bounty and arriving on Hachinosu. There he crosses Whitebeard and has his first contact with the Pirates Rocks, of which he will later become one of the most famous members. After the defeat in God Valley, the crew disbands and Kaido starts recruiting people. He meets King and Higurashi Kurozumi, with whom he forms a pact. Then his reign over Wano with Orochi begins to the present day where he awaited the arrival of Joy Boy.
There are still gods to tell dark points in ONE PLAYwho knows if Eiichiro Oda will continue to follow this emperor's past.