How does an ordinary girl end up with superpowers?

For Etsuiku it happened when she was kidnapped off of the streets during Halloween when she was only 12 and injected with 'monster blood'. It was supposed to give her superpowers to kill 'the real monsters'.

She knew that it was nonsense even at the time; it's not how blood transfusions work.

So why did it work?

Nanites.

Whoever's blood it was that they transfused into her had nanites in their body. After 48 hours a message appeared in her mind's eye like text on a screen.

Emergency activation of Sword and Shield System.

Password?

She was too desperate and scared to question it. Strapped to a bed, the teen hoped that she wasn't hallucunating and whispered, “Silver Star.”

Pain slammed into her and she didn't remember anything else until she woke up in a hospital. The nurses were very kind and gentle to her and she wasn't overly surprised when they wanted to run some tests.

Nothing came up, not even the nanites.

It left her to wonder what had really happened to her that night.

She had called her parents, who had come and got her, worried sick. They had cried and hugged her and swore off ever doing Halloween again.

It was a promise that they had stuck to and, truthfully, Etsu was grateful for that.

As she recovered Etsuiku found that physical activity helped to alleviate the on-going nerve pain that she had developed in the wake of whatever it was that had happened. After a couple of weeks she realized that she was now scared of people. Not wanting to feel that way for the rest of her life, Etsu made a decision and asked her parents to enrol her in a martial arts class.

After several weeks of searching for some kind of school that didn't 'guarantee' a black belt by a set amount of time they finally settled on basic self-defence classes and parkour.

Thank God it was summer vacation by then.