[@VitaVitaAR]

I can think of a few prompts for Kazue from normal to outlandish. Just throwing stuff out there, though she has a lot of the character traits that Song has already exhibited in many ways, which could actually be something to work with. (Quiet, incredibly lethal and precise, likes cute things)

Normal - Grew up training, developing a strong sense of justice. In her culture (Not necessarily Japanese in this world), 16 is the age of adulthood, and all children take a journey to see the world. She decided to devote her journey to traveling to hone her craft and maintain the balance of peace. When she found how the world was however, she abandoned her home country and joined the navy.

A Bit Wack - Daughter of a strict, almost abusive family renowned for their sword arts, but embraced her upbringing and became a protoge. Has lived her life by the straight and narrow, to the point of naivety. Was tricked into leaving her home country by a human trafficker/slaver/shady dude who promised [something]. When she arrived and found out what things really were, she fought her way out, realized there was no way back home, and decided to enlist in the navy. She still carries the scars from her upbringing, though she does not recognize them. 

Outlandish - Like Song, first memory is waking up in [some dark place] with the name "Arakawa Kazue" scrawled in [blood] over her head. She found that (like Song) she has insane sword skills, and also has a related useless craft skill like [making paper lanterns]. She also stumbled into the city looking for answers, but instead of finding a Jonathan, she found the Navy, who took her in. Her parallel origins might explain why her personality highly resembles Song's. (Actually, this route opens up an entire back storyline in Song and Kazue's origins if you're interested. I'm kinda hoping you're interested, but it's cool if you want to write your own thing. PM me if you are interested in hearing more though.)

I might be taking this too seriously...