Phantom Blood - Chapter 7
Seven years before the current story, Dio is seen trying to apply medicine to his father Dario. Dario is unwilling to take the medicine, ordering Dio to sell his deceased mother's dress and buy alcohol with the money. In reaction to this, Dio cries and resolves to kill his father, and is seen buying oriental poison from a man in London. Though Dio is wearing a mask at the time, the man selling the poison takes note of the three moles on his left earlobe before selling him the final dose.
Back in the present, Jonathan has realized that Dio killed his own father, Dario, and is trying to use the same poison to kill George. When he is confronted on the matter, Dio denies Jonathan's claim, but Jonathan says that he plans to investigate the legitimacy of the medicine Dio has been providing for George. Jonathan snatches the paper package carrying the medicine off of Dio's tray and refuses to return it. Dio threatens Jonathan, telling him their friendship will end if the latter will not return the medicine. Jonathan is unable to meet Dio's eyes because he knows that his claims are based entirely off of a hunch and have no solid proof. Jonathan promises to return the medicine and not speak of the incident if Dio swears his innocence on his father's honor. As Jonathan expected, Dio angrily refuses, stating that someone like Dario never had any honor. The thought of Dario enrages Dio and he punches Jonathan; however, years have passed since their previous brawl and Jonathan has grown much stronger. Jonathan has resolved to protect his family, grabbing Dio and throwing him down through the railing from the second floor of the mansion to the first using only one hand.
Jonathan knows that he and Dio were never truly friends, and says that he will make sure to get the medicine analyzed and send Dio to jail. Dio knows it will take Jonathan at least three days to find proof of the poison, due to it being from the east, and determines to finish off Jonathan before then.