She is the other upper moon 1 (she and Kokushibo are the same situation as Daki and Gyutaro were as in they have to be beheaded at the same time cause they are both upper moon 1)
She became a demon against her will and she hates fighting.
Basically Michikatsu (Kokushibo) wanted Yoriichi to lose what he cared about more than anything, Milo.
So when he was given the chance to become a demon, he took his 14 year old little sister with him.
As stated before, Milo hates fighting so Kokushibo insisted that they be fused together so she would benefit from his energy and vice versa. (is that how it works? idk honestly, im fr just making things up at this point.)
Also, Milo was born with the demon slayer mark like Yoriichi. She created the star light breathing form.
So Yoriichi is the sun, Michikatsu is the moon, and Milo is the stars.
She was the star light hashira in life.
I’d be happy to explain more about this au if anyone wants to hear more about it!:3