Throughout his years at Hogwarts, Regulus was marred by increasing tension between his family and his brother, Sirius. Sirius' membership in the Marauders, a group of students consisting of several Gryffindors, led him to treat Regulus terribly, which pushed him to form friendships with Severus Snape and other Slytherins. His elder brother's increasing prejudice against Slytherin house and treatment of him led him to alienate his parents and eventually run away, crushing the Blacks and leaving Regulus to deal with the fallout. This led to Regulus hating his brother and blaming him for his mother's suicide attempt and father's descent into alcoholism.
After graduating from Hogwarts, Regulus married his girlfriend Mary Macdonald, a Gryffindor student and best friend of Lily Evans. With her, he had two sons; Julius Marius and Paulus Phineas Black. After Sirius' abandonment of his pregnant girlfriend, Marlene McKinnon, she was taken in by Regulus and his wife and he helped raise her daughter, Urania. After a few years, with the consent of Mary, Regulus took Marlene as his second wife.