Where it all began
When regulus was growing up he would always look up to Sirius. Sirius was always there for him even when regulus wasn’t there for sirius, truly regulus thought he was his knight and shining armor but when he started to get older things changed. Walburga was always a stone cold person, even when she was younger even her parents got scared sometimes, so when she had children it was a shock to all since how could this one person be capable of loving children. Of course only the outside people thought that as all the pureblood families knew what goes on in their houses. Some people assumed but could never prove it since in public you would only ever see them acting prim and proper. You would never catch a respectable pureblood out of line, well that was until she had Sirius. He was the heir, the one that was supposed to be the golden child, which unbeknownst to her he really did wear gold well, as well as red. So when Sirius was younger and she could tell he was not going to be the perfect child for the black family she had regulus. Of course regulus followed the rules and did anything she said to not get in trouble. Except he didn’t. But she didn’t know that since Sirius would always cover for him even if they were fighting it didn’t matter if regulus did the worst possible thing, they were kids and all Sirius knew was he didn’t want his little brother getting punished so he would take it for him.
As they got older things changed. Sirius went off to hogwarts and there was no one left to protect him. So when he would misbehave he got the punishment, and it was always worse when Sirius left since she had no one left to take her anger out on, it was worse when Sirius didn’t come home for Christmas break, she was absolutely furious with him and Sirius knew it. Sirius wouldn’t go home knowing he would get punished horribly for getting sorted into Gryffindor even if he told him friends differently and that he just wanted to stay at Hogwarts to sneak into the kitchens without getting in trouble. When the school year ended and Sirius came home the first thing he saw was regulus looking so relieved to see him, but when walburga came in behind him something changed. Regulus changed. You could see it in his eyes. If Sirius was being honest he didn’t want to believe that regulus wouldn’t care for him anymore but once walburga started to crucio sirius for all that he did and saw regulus leaving the room with a straight face he knew something terrible had happened to his little brother for him to not even make a face at what was happening.
When the year for regulus to go to hogwarts came around he was terrified. He knew he wasn’t courageous like his brother and even though they would both be gone now, nothing would change. He knew that he would get in Slytherin and the little ounce of hope sirius had left for his brother would be gone for good. When regulus got in the great hall to be sorted all he could hear is his brother laughing oh so loud with his friends. And all regulus could think in that moment was Sirius never looked that happy with me.
When regulus got in Slytherin no one was surprised. They all saw it coming. When Sirius would act out, regulus would be on the side line in the corner just watching. Even if it looked like regulus wanted to be doing that too he never would. He knew what was going to happen when everyone left and Sirius is left alone with their parents. He could hear him screaming in agony from his room every time. And even though Sirius would come to regulus’s room every time after and reassure him that he was okay and not to worry, he always did. He knew that Sirius would lie to him to protect him from the truth. The first time sirius didn’t go to regulus’s room after getting punished was when they both got home from hogwarts, that was when regulus knew everything changed and there was no longer the duo of Sirius and regulus, even if it was only them who knew that duo existed.