Regulus grew up doted on at every moment because he liked to follow the rules. He could get just about anything at the snap of his fingers, besides what he really wanted. All Regulus wanted was a family. Regulus's parent's were cruel people even if they did spoil him; they didn't do it out of love, they did to make sure he never had a reason to not obey them. The only chance he had for a family was his brother, but even that fell short by a mile.
When Regulus was younger he and his brother practically were attached at the hip. They did everything together, from brushing their teeth to running the country side. You could never find one without the other. Sirius once beat up a much older wizard who dared to mention to Regulus that his front tooth hadn't come back in yet. That all changed when they got older. His brother made a new friend and started veering off to do his own thing. He also started breaking the rules and getting into trouble more often. His parents separated the brothers to insure that Regulus's perfect behavior would never be spoiled.
Then Sirius went off to Hogwarts and broke more rules than Regulus had even heard of. He felt lonely and sad; soon he became very depressed so his parents intervened. They made all of their colleagues bring their children to the house, but Regulus never grew attached to them. Three stubborn children became his friends and helped to build him back up again, bringing back his smile every once in a while. He liked their company but he was never sure if they liked him or if their parents told them to be his friend. Most of the time he didn't care.
The next year he went to Hogwarts and he actually started to feel happy again, but all that stopped when his brother turned sixteen. He decided it would be cute to write a letter home about why he wouldn't be coming back that holiday. His mother visibly flinched when she read it, and she broke down.
"My son is trying to kill himself." She cried. "He disowned us for a dangerous and filthy monster."
He didn't get to read it as she had burned it, but he later found out what it said: I won't be coming home for Christmas as I will be too busy shagging my half blood werewolf boyfriend- Sirius Potter.
He remembers the fight that truly lost him a brother vividly. Sirius had come in the door the summer after he sent the letter smiling, and their mother stopped him in the doorway. She started shouting about dangerous animals and filthy half bloods. Sirius fought back saying she was cruel and prejudice and she slapped him and sent him to his room. He left and didn't come home ever again. Regulus's friends had to stay with him all summer, and they only managed to make him smile a few times.
Then in Sirius's seventh year he heard something that made his heart break in a way that never quite healed. He was on his way to the library walking about 20 feet behind James Perfect Brother Stealing Potter.
"Brother! Brother!" shouted Sirius; he turned around a glimmer of happiness bloomed in his chest and saw his brother running up holding a bit of parchment. "James!" he shouted running right past Regulus as if he wasn't even there. "Mum and Dad wrote us a letter!" he said as easily as though they really were his parents. Regulus stopped dead in his tracks, and slapped his hand over his mouth to keep from saying something. James was glowing over the fact that he successfully stole his brother from him.
"What does it say? It says we got the apartment!"
Regulus never made it to the library that day.
Then came the day that Sirius made it clear to him what he really thought of him. Regulus tried to stay in relative contact with his brother but he found it hard, as he had a perpetually full schedule and little time for his low priority forgotten brother. It was a Friday and Sirius had only popped in for a minute before he had to leave for a double date.
"Do you want tea I just made it?" Regulus asked moving to poor it.
"You joined them?" Sirius asked loudly, standing up. Regulus jumped and spilled the tea, looking startled. He sighed and started cleaning up.
"What else was I supposed to do?" he asked softly after a long silence.
"You could have come to me." shouted Sirius
"When?" he asked loudly making his brother do a double take. "Before we got separated and moved into different houses because of your behavior? Before you went to Hogwarts and forgot all about me? Before you sent that letter to mum-"
"It wasn't me who sent it! I wrote it as joke and Peter sent it!" He protested.
"Or after you ran away leaving me with them alone? What about when you decided to pass me in the hallway like I was a ghost shouting brother to that smug shit eating arse to talk about your fake family?" Regulus was close to tears and livid, visibly shaking.
"This is about James? You're mad that his family loved me so you decided to join the death eaters?"
"You have no right to demand answers from me acting all righteous! You have no right because you got a life! You got a family! All I got was an awful set of parents and an absentee brother! I finally found a family! They'll kill for me which is more than you've ever done! You can go get shagged by your filthy half breed boyfriend you pouf!" he screams tears flowing. Sirius gave him a "you're dead to me" glare and left.
The following week he betrayed the dark lord and his dark adopted family for his absentee brother by stealing a locket of great value and trying to destroy it. Regulus's friends saw him only one more time after that; at his funeral.