Yumi's was born in the soul society as a soul reaper but because her mother was a half soul reaper and half hollow so she turned out to be a half soul reaper half hollow. Yumi's mother was executed publicly. With no father she became just a stray till a girl her age (5) Named yoruichi another stray became your friend. You two became inseparable nobody knew about your half hollow side. You 2 got accepted as actual legit soul reapers. You climbed the ranks higher and higher. She became a captain at the age of 30 (while still looking like a teen) you were her lieutenant same age.