Am I About to Join a Kill-Or-Be-Killed Love Triangle?

Kohaku is a girl who spent her junior high school years as a loner and dreams of living a fulfilling life in high school. However, on her first day of school she falls down the stairs and ends up hospitalized for a month! This means that everyone besides her has already formed a group of friends at school... In order to avoid being a loner, she summons up the courage and calls out to the only odd-numbered group in her class, but it seems that they are high up in the school caste system, and are actually involved in a blood-splattered love triangle with a non-human...!