Deep in the ocean where no human is able to reach even with today's technology lives a type of bizarre marine animal, evolution gave them the ability to change form and learn the behaviors of other creatures to mimic them, this is to lower the prey's alert and attack when they are relaxed. They only need a few hour to a few days to change their form——from appearance to internal organs—— dramatically, and their strong ability to adapt to all kind of habitat is never seen in any other creatures in the world. The story begins when one of these curious creature swam into shallower water by chance and saw human.
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In a chilly autumn morning, an marine biologist found a weird looking person lying unconscious by the shore: the person have a large, mouth looking opening with long, sharp teeth between her legs; slippery yellow tentacles at where the hands and arms are supposed to be, and a lightbulb-like thing hanging on her head. The scientist quickly realize this blue hair girl might be a ground-breaking discovery in her field of study, and brought her back to the lab. The name "Angler" is temporarily given to this mysterious creature as the lightbulb-like part on her head reminds people of anglerfish.