Seven is an experimental subject created by scientist as a weapon. The origin of her is that she is a child which who is infected by a reptilian virus. So the scientist make her as an experiment to find the cure for her disease by using a lizard DNA. The experiment was a success but the side effect is she grow a tail and poses power outside of human capabilities.
Her family rejected her and she was left with all the scientist. Because of her tail, she was not suit to go to school like normal teenagers on her age. When she was 15, a group of organization bought her from the scientist to free her from becoming an experimental subject for the scientist.
In the organization, she was trained to become an assassin. Her code name was Seven, as she was the 7th member of the organisation. Until now she has no idea what the organization is all about, because she was only doing what they told her to do so. Everyday, she went out at night and search for her target to and assassinate in silent.
I guess that How Seven become Seven XD
The end
