Okay sorry for being inactive I was asleep but here's why Yuki wears her headphones and is hostile towards people.

Okay since Yuki was about 6 years old her father started to come home drunk and beating her and her mother up. Once her father was sober her parents would fight and that's all she could hear. Around the age of 10 she would start lashing out at her friends and that's when she started to wear headphones all the time. When she was home and her parents were fighting she would cry in her room. And basically everything for Yuki and her family went downhill yet her parents won't get a divorce because they don't have a lot of money.

Poor poor Yuki...