Aqua Kingdom

Tastumi is a normal boy going to school and living with his strict older brother. Orphaned at a young age, his brother has been both a mother and a father to him. One day, he and his friends are about to rescue a beautiful girl from being harrassed when she beats them up herself. When she sees Tatsumi, she suddenly calls him "Rui". What does this mean? Who is the girl? Why are there strange men attacking him all of a sudden? And why is his brother suddenly so different??