Long ago i had the theory that Kaido offered small island to spare their live and treasures, if they give them their strongest young fighter. I also thought Jack was getting into Kaido crew that way. And when he was at the Dojo, first Kuinas father refused, but she kinda insisted, cause she thought it could be a way to become stronger that way and maybe could be a guardian for the island in the future.
So she joined Kaido. She additionally then got a DF and changed her focus on the DF, cause it made sense for her to get rid of the stigma of a female, so she could get stronger then man by being a beast, not a man or a woman. This is also why she is hiding her face, cause it would reveal her real face and her gender, which isnt part of her personality anymore. She doesnt see herself as a woman, also not as a man. She is "King", without any further declaration.
And i think her becoming a beast will finaly the one weakness she has to offer, so Zoro is able to beat her, because of a weakness of the Zoan DF she is using. And at the end, when Zoro is able to beat King/her, he will be kinda mad. Kuina will fall back in her "woman"-"man"-Theme and declare she couldnt beat the one man she always wanted to beat. "You beat me now Zoro, are you happy?"
Then Zoro throws her sword to her. He looks at her, angry.
"I could have never beaten the swordfighter you are."