Ryuuji Yashiro is the boy also known as Chaos who controlled people through a cellphone game. He's the Yin Vessel of the Golden Dragon.
He's a very uncaring person who, basically out of boredom, killed Tatsuma Hiyuu's adoptive parents. He said that he's unable to feel anything, even if he tried to do something about it. In front of others, he fakes emotions like concern and sadness and he has no problem hitting other's soft spots, seen when he even brought up the death of Tatsuma's parents, knowing that he was the one responsible. He is however shown to be able to feel anger, more specifically at Tatsuma.
He's first introduced as a wheelchair basketball player with a girlfriend. Some time later, Tatsuma saves him from getting hit by a vehicle. Through the use of his powers, he had Tatsuma's parents killed. He said that he would observe Tatsuma's group of friends and also that he chose Tatsuma because he seemed the happiest out of all of them. Later, Tatsuma is shown taking him to places around Tokyo. Munetaka Yagyuu is the one who uses his powers near the end of the series.
With Tatsuma: When Tatsuma first heard the news, he was clearly angry at him for the murder of his adoptive parents. Despite this, he was somehow able to let go, later saying that he doesn't know if he hates Ryuuji or not. After the fight with Yagyuu, Ryuuji seemed to have been looking forward to Tatsuma making laugh since that's what Tatsuma promised he'd do.
To Tatsuma: "You do hate me, right?"