Satougashi no Dangan wa Uchinukenai: A Lollypop or A Bullet

Yamada Nagisa is a bored teenager who finds everyday life extremely unappealing: thus her plans to join a military academy as soon she graduates. On the last term of middle school, a strange girl named Mokuzu Umino, who claims to be a mermaid, transfers into Nagisa's class and, much to her bewilderment, begins to follow her around incessantly.