One day she was headed to school, but her parents were missing.
A guy in a mask came to her house, and said he was going to help her, and her little sister begged her to come. She saw a flash of red in her sister's eyes, and realized that it wasn't her sister, it was a monster.
She followed the guy in a mask to his HQ. It was unbelievable.
The ninja took off his mask and introduced himself (Leo) and the other people in the room: Ben and Lissa
They explained that they went to a training camp to become monster fighters, and protect the mortals, and they wanted her.