Aren't you curious of what Mikori did when she was little, and how she became the strict, disagreeable girl she is? Well, let me tell you now.

Mikori is exactly 169 years old, which only makes sense because the residents in Soul Society age slower than humans.
She lived in Rukongai District 37, on the south side. Not too shabby. Her parents were plain boring, and were the owners of a somewhat known restaurant in town. Mikori had an older brother, and her older brother was a monster.
Her brother Youhei was the family's "gem". He was talented and "handsome". The family thought that Youhei could become sucessful as a shinigami. Youhei was 6 years older than Mikori, and never treated her with respect. He would push her around when she was a toddler. Youhei acted like he was superior; he treated Mikori like his slave. It got progressively worse as they grew up. Their parents never found out no matter how many times Mikori told them, Youhei being a kiss-up. She received the blunt end of the stick, and was not very educated in things.
Mikori didn't get along with other kids well, because she was indeed bossy. All her anger and frustration with Youhei vented out in her personality. Whenever her parents and Youhei were gone, Mikori would act prissy- like she was the queen of everything non-Youhei. But when Youhei came, there was weakness that you could see in her eyes.
Mikori became part of the "popular" group of kids in town. She was demanding, and cowards would hide under her for protection. The "popular" kids in town never turned out good. She did things she shouldn't have done- and never would've done, if her mind was clear.
Mikori spent the majority of her time hanging out with her "friends", but in reality, they were nothing more then just fillers for her time. She didn't want to deal with Youhei or her ignorant parents at home. She fought with other kids many times and would come home bruised more often than not. In secret, she would teach herself how to read and write with Youhei's books. Once, she was caught by Youhei- who locked up his books somewhere. Mikori never learned how to write more than small words like "city, ramen, signal," and "reach". Currently, she still struggles with writing,which isn't the most beautiful.
Mikori grew up with this life, yet her personality got better, but not by much. She wasn't as power-hungry as she used to be, but she wasn't as nice as she pretended to be, either.

Youhei grew up and indeed became a shinigami. He graduated, top of the class. He later died in a mission in the real world, and hollows attacked. That's when Mikori herself started training to become a shinigami. She had a flare of arrogance that came up when her brother died. Now, she was completely whole. Mikori never talked to her parents; she felt that they ignored her abilities for most of her life, and so deserved nothing from her in return. Even now, Mikori doesn't know if her parents are alive, how they're doing, or anything about her family.
She avoided all her friends and family when she was accepted into Shinigami Academy.
Her life after joining the Academy is a different story altogether...

Since the bottom image uploader isn't working, I'll just be ghetto and post links of a picture of what I visualize Mikori's younger self looked like.