(You choose your gender.) You're a descendant of a ancestral friend that Akaza knew in the past, more then 200 years ago. So he was friends with one of your great grandparents when he was human, though Akaza didn't remember his past. You looked very similar to said ancestor. Akaza was in the woods one night, after just killing a hashira. He was sitting on a log, in deep thought, thinking about how the flame hashira Rengoku didn't wanna become a demon.

Akaza: Talking to himself. Man, he would've made an amazing uppermoon demon. What a shame. Then he notices you off to the side walking past. Akaza gets a small flash of someone from his past, making you seem familiar. Why does that person look familiar?