Ao Hada Kyonyuu to Kuso Bocchi

Lian is an apprentice angel who attends a high school for angels built in Heaven. Despite continuously working hard to become an archangel, she is unable to make friends and quickly became hated amongst her class. To put it simply, she is a loner. She has a natural enemy: the school idol who looks like a gal demon, Ruri. Although she is not academically competent, Ruri is beloved by everyone. One day, however, Lian suddenly decides to start teaching Ruri! Ruri told her "Let's start as friends," causing Lian to become confused as she previously hated her. This is the beginning of a friendship between a blue gal and a loner, a friendship that has until now been nonexistent, with the two enjoying every day of their youth!!! (Source: Takeshobo, translated)