Sirius Black: former Marauder and godfather to Harry Potter. Ran away from his family at age 16 after becomming dissolusioned with their belief in Blood Purity. Wrongly accused of betraying his friends, the Potters to Lord Voldemort and killing Peter Pettigrew (the real traitor) and 13 muggles. First to escape Azkaban after realising where Pettigrew was and showed Harry Potter and his friends that he was innocent and tried to kill Pettigrew. Pettigrew escaped a second time, so Sirius's innocence wasn't proven to the ministry until after his death at the Battle of the Department of Mysteries.
Despite them choosing different paths when they were young and having opposite views, there are some interesting aspects of their lives that they had in common:
Both can be considered traitors: Sirius was openly against his family's traditional views and ran away when he was sixteen. Regulus secretly betrayed Lord Voldemort after he realised that Lord Voldemort had both tortured his house elf and created Horcruxes.
Both are discovered to be better than Harry/the reader thought they were before: In the prisoner of Azkaban, Harry finds out that Sirius Black was always loyal to his parents and that it was Peter Pettigrew, not Sirius that both betrayed Harry's parents to Lord Voldemort and killed thirteen muggles. In the Deathly Hallows, Kreacher explains that Regulus betrayed Lord Voldemort (after realizing that the latter had left Kreacher to die after testing the defenses of his Horcrux) and managed to obtain the Horcrux, albeit loosing his life during the attempt.
Both died without having male (or any) children, thus rendering the Black Family tree extinct in the male line.
Both of their bodies were not burried after their deaths. Sirius died by falling through the viel in the ministry of magic and so his body was no longer in the world of the living. Regulus was drowned by the Inferi in the Horcrux Cave and his body was not recovered
Both of their deaths could have in theory, been avoided: The Battle of the Department of mysteries happened because Voldemort sent Harry fake visions of Sirius being tortured and Voldemort would not have known to do this if Kreacher hadn't told the Malfoy Family about Sirius's relationship with Harry. If Sirius had been nicer to kreacher, Kreacher wouldn't have fled to the Malfoys. Sirius's actual death in the department of mysteries was caused by him taunting and underestimating his killer, Belatrix Lestrange. Regulus Black told Kreacher to leave him to die in the cave and Regulus drank the potion himself. He could have brought water with him and told Kreacher to dissaperate him to safety.