Part 1. The beginning: Misono Mika was once part of the Tea Party the Student Council of Trinity. The Tea Party wanted to make peace with a school known as Gehenna everyone except for Mika wanted this to go through instead she wanted to make peace with one of Trinity's old Branch school known as Arius. Mika did eventually make contact with Arius though one of their students. Mika bemoaned the fact that she was not the Host of the tea party (The member who is the leader of the tea party) And Saori the student who Mika made contact with exploited this insecurity and suggested to take care of the current Host of the Tea party Seia. Mika thought it was a great idea but little she did not know Saori had...a hidden agenda,Arius was at war with Trinity along time ago and they lost and wallowed in hatred and had a strong desire for vengeance and Saori thought it would be a great opportunity to assassinate Seia. Mika was unaware of this and thought that she and the Arius students will go to Seia's apartment and tell Seia that Trinity and Arius can work together. Mika's plan did not go well and Seia was taken out of commision and in the chaos Nagisa the other tea party member became the Host.
TLDR: Mika who hates Gehenna anted peace with Arius. Arius exploited Mika's insecurities to assassinate Seia the current host of the tea-party, after the chaos Nagisa becomes Host.
