Riya Narukawa is a high school freshman with amazing talents for the violin but also suffers from mental issues. Riya has a multiple personality disorder making him seem perfectly fine and good one day whereas another day he can change into the complete opposite. There are "two" Riya's. He calls his other self his brother and thinks that his brother is inside of his body forcing Riya to play the violin. The "two" Riya's are also known to be called Black and White. The multiple personalities give Riya a lot of complication and distress, but when Riya meets Atsushi, it seems that his life begins to change.
Later on in the manga, Saki (Riya's cousin) explains to Atsushi that Riya has been abused by his father ever since he was young. When Riya was in middle school he and his friend Yasui ran away from home to get away from Riya's father. Yasui then got hit by a car and suffered a broken arm which enabled him to play the violin. Angered, Riya's father beats Riya up to the point he blacked out and had to be hospitalized. After awaking from his concussion, Riya seemed to be a total different person. The incident with Yasui and his father seemed to have triggered his split personality.(Source: Mangafox)