Ryuu is very intelligent, and his dark background somehow links him to the organization known as Pluto. When his parents died, Pluto took care of him. As a child he was never allowed to socialize with others. He often viewed his house as more of a prison than a home. Even with his cold personality he is still friendly with the other members of Class Q, and shares a special link with Kyuu since he was the first one who could understand him.

(Source: Wikipedia)