He was in an abusive relationship that lasted 2-4 years. His partner insisted that he couldn't trust anyone but them, and that everyone else who said they liked him was lying. This partner also constantly played with his feelings. Telling him that they loved him and always would but then ditching them whenever someone new popped up. Eventually Yukii grew numb to it. He's used to being used and having his feelings played with to the point that he's grown unresponsive to the concept of 'Love'
He's a Bunshiba. Half bunny, half Shiba Inu.