Hekikai no AiON

Tsugawa Tatsuya just lost his parents in an accident... but he's trying not to let that keep him too down as he starts highschool.While trying to obey his father's final wish for him to become an upstanding man, he ends up helping a girl who was being bullied. Despite her exceedingly cold nature, he just can't leave her alone... which may not necessarily be to his benefit, as he ends up getting wrapped up in a world where things like parasitic mind-controlling bugs exist... as time passes more mysterious events keep happening one after another: mainly the long hateful battles between mermaids? and the witch?