Kaido is a Yonkou, aside from that we know nearly nothing about him. Now here's my speculation. Kaido is a former Admiral who left the marines and became a pirate. The incident took place some 10-15 years ago. At that time X-Drake was a Lieutenant/Captain and worked directly under Kaido, that's why he has a hatred for him as Kaido was not a generous superior. As a former Admiral, he is insanely strong and therefore when he became a pirate he could easily secure the position of a Yonkou. How's the idea?