Yuui-13 is actually a top secret weapon of the government's, the government keeping her a secret. Although as time passed by, she soon escaped the government's clutches because she developed feelings after killing so many innocent people and agents in the process of being tested as a weapon. So she flees to a city by flying, where she crashes into a river bank. A nice guy and his little sister so happen to be heading home from grocery shopping when they saw her crash into the river, so the guy rushes down and jumps into the river to save her. He and his little sister carry her to their house and let's her rest there until she wakes up. After Yuui woke up, she had explained everything about herself and had intended to leave right after telling them her story. Although, after they met her they thought she is actually a nice person, so they had decided to let her live with them. So she agrees to live with them and plans to attend high school to live a normal life, well that's what she wants anyways. However, Yuui does not know much about humans and how they act, so she has to learn many things about them and their lifestyles to be accustomed to society.
-Her mini wings actually grows into bigger wings, such as this:
http://cache26.fansshare.com/photo/angel/shutterstock-angel-wings-wings-1264672943.jpg
-Her wings are light as a feather, but is actually as hard as steel. She can harden her wings to feel and be as hard as steel, but during most of the time, they feel as soft as normal feathers.
-Her mini wings actually grows into bigger wings, such as this:
http://cache26.fansshare.com/photo/angel/shutterstock-angel-wings-wings-1264672943.jpg
-Her wings are light as a feather, but is actually as hard as steel. She can harden her wings to feel and be as hard as steel, but during most of the time, they feel as soft as normal feathers.