Dio was the son of the late Dario Brando, a poor thief and drunkard, who poisoned and killed his father out of retaliation for making his mother suffer until she died and found out that his father planned to send him George Joestar I, a member of the upper class who believed Dario to be his rescuer during an accident, in order to raise Dio as if Joestar was his own father if Dario died.
When arriving in the Joestar grounds, Dio immediately tries to assert his dominance and attempts to intimidate George's son, Jonathan Joestar, by kicking aside Jonathan's dog in disgust when it was approaching him during his arrival and manhandles Jonathan after Jonathan attempts to help him with his luggage. He further alienates Jonathan from Jonathan's father and the local boys for the next few days by performing better manners in front of George than Jonathan did while brutally humiliating Jonathan in a local boxing match in front of the local children and started spreading around rumors of Jonathan not being capable of keeping secrets.
After seeing Jonathan being happy with the a local girl known as Erina Pendleton, Dio forces Erina to kiss him on the mouth in front of his friends and immediately attacks her after seeing her wash her mouth with mud as if to say that Dio was more disgusting than mud.
Dio is then confronted by Jonathan after Jonathan heard from the local boys what had happened between Dio and Erina. Dio, at first, had the upper hand during the fight but is then overwhelmed by Jonathan by headbutting and throwing several punches towards his face, knocking down Dio. Dio's blood sprays across the room, with some landing on a stone mask, an artifact collected by Jonathan's late mother, hanging on the wall. This causes the mask to grow several rows of curved spikes from behind, making it fall on the floor and catching the interest of the two children.
Dio then proceeds to attempt to attack Jonathan with a hidden knife but is stopped by Jonathan's father, who saw the fight happen and sends both Dio and Jonathan to their rooms.
Several years later, Jonathan and Dio grow up to be excellent students and athletes but the latter still holds a grudge towards the former for what happened in the past and merely pretends to be a good sibling to Jonathan.