Child by Rape: Ralph's father raped Sakuya's mother, then Sakuya was born from it.
Compensated Dating: Did that ever since he was in his late teens. The manga takes it further, making him a straightforward High Class Call Boy. This is how he met Ayako Sakura, actually.
Financial Abuse: Was adopted and raised by Reiko's manager after her death, but mostly because said manager saw that Sakuya had inherited his mother's musical talents and wanted to use the kid to Work Off the Debt that Reiko left before dying.