Pholus and Trisha Edit
Usotsuki was in love with Pholus, despite it being unrequited. Pholus was more interested in caring for his younger sister, Trisha. Trisha saw Usotsuki as a type of older sister, and enjoyed being with her. Pholus disliked this and saw Usotsuki as a bad influence towards his sister. (Porcelain Doll effect having a positive reaction with Trisha but not with Pholus)
Usotsuki eventually realizes that Pholus has no interest in her, and decides to kill Trisha in order to steal his attention. Enraged by this, Pholus gives chase.
Usotsuki is not a real name Edit
According to the meaning of the name Usotsuki may be called differently, the name that we know is may be as her nickname.