Regulus Arcturus Black has always been different. Ever since she was born, she's been different. Nobody knew that, of course, except everyone did. They knew it, but they couldn't prove it. Perhaps it started when Regulus's dorm room was wrong. She was elated, but no one knew. Everyone said it was a mistake in the magic, and they all chose to believe it. Even though Hogwarts doesn't make mistakes. That wasn't the problem, though. The problem was when it happened again, and again, and again. I'm sure people thought it a bit strange how Regulus didn't seem to mind sharing a dorm with the girls. Some of the older kids said it was unfair and whined about how lucky that kid was. Regulus thanks Merlin that she is of the Noble and Most Ancient House of Black. This way nobody questions her.
Here's a little bit of context. Walburga and Orion had two children: a daughter; a son. Sirius and Regulus. Sirius was also different and was never one for conformity. From the moment he could understand what gender was he was protesting the one they'd assigned him. So when he went to Hogwarts and the stairs wouldn't let him up, he flaunted to his parents how he was right and if Hogwarts believes that's good enough for him. Everybody at the school knew him as the son, so they were forced to go along or risk being exposed. They already knew Sirius wasn't going to be what they wanted; nobody was expecting Regulus to backfire on them. It was just a small hiccup that they smoothed over quickly. Besides, the Black brothers kind of had a ring to it. But Regulus Arcturus has always been different, even from her brother.
Regulus knew there was something wrong with her. Orion and Walburga tried their best to make sure no one found out. If nobody knows, then nobody cares. Appearances matter. It's the first thing someone sees and the first thing they judge you on. It's why she isn't bothered, she appears perfect. Under the surface, however, she is far from it. You see, her brain is wired something strange. It makes her clothes itch and her head ache. It makes Potions too much and Quidditch games torture.
Because of it, her parents will always be disappointed in her. She may be better than Sirius, but that isn't much of a feat. Since she was little, she was always misbehaving. Moving too much, making weird sounds, taking things far too literally. Regulus didn't speak a word until she was three and a half years old. Sirius wasn't much different in terms of good behavior. He never sat still, did things he knew he shouldn't, focused none of his studies. He opened his mouth at nine months old and hasn't shut it since.
Okay, maybe they weren't too different after all. That was the problem, though. They weren't different, so why were they treated that way? Why did Sirius get away with bouncing on his heels when Regulus couldn't even shake her hands? Why was he allowed to click his tongue, but she wasn't able to hum? It wasn't fair, and when he went to Hogwarts it got even worse. He was always her Siri, and she was his Reggie. When he left, she was alone. See the thing about Regulus is that, much like Sirius, she doesn't like being alone. However, Sirius will latch on to anybody while Regulus is extremely reserved. They always had each other, then Sirius made friends and Regulus didn't get attached.
That brings us to our current situation: secluded, deep in the restricted section of the library rocking back and forth and back and forth and back and forth. Hugging her knees with her teeth in her arm to try and keep as quiet as possible. How did she end up here? It was a bad day. Her clothes were too itchy, too many people called her "sir", everything was so loud, and three students bumped into her all before her third class of the day. She skipped breakfast because she couldn't breathe in the Great Hall, she almost threw up in potions because of all the smells, and her partner kept touching her even after she told them to stop. She tried to go to lunch, but couldn't even step her foot in. All the teachers kept calling on "Mr. Black" when the only reason she wanted to open her mouth was to scream. Finally, the day was over. All she had to do was eat something for dinner. It was worse than breakfast. To her credit, she tried. She almost sat down at the table. She could've made it, maybe had some buttered toast and a glass of water. Unfortunately, some idiots were messing around. She was just trying to get to her seat when one shoved the other right into her, spilling something down her robes. Now she's exhausted, hungry, sticky, completely overwhelmed, and would curse someone for a glass of water.
One problem with the library is curfew. Luckily, that wasn't for a few hours. The other problem with the library is people. As restricted as this section is, Regulus still has a pass. Which means she shouldn't have been surprised to see her brother. He didn't see her, thank Merlin, but he heard her. The curious expression on his face as he paused to listen better. As he looked around, she pressed herself tighter into the corner, squeezed her eyes shut, clamped her hand over gritted teeth, and prayed he didn't find her. When she opened her eyes they were locked with his. He quickly surveyed the situation, closed the door fully, and sat on the floor in front of it.
She had only been in the library for fifteen minutes and arrived in such a state that she had forgotten to take her usual precautions. A silencing charm on the closed door to a secret room in the restricted section of the school library. Nobody ever comes back here, ever. Regulus was confused, waiting for her peace to be broken; it's still quiet. He's never this quiet. She removed her gaze from the floor to find Sirius in a similar position: back against the door, eyes closed, hugging his knees. Except, he didn't look stressed. He looked peaceful, serene. It was weird. Whatever, she didn't have time to figure out her brother's stupid antics; he doesn't make sense to normal people, and she's been caught on a bad day.
Actually, she didn't want to admit just how much this was helping. She didn't realize how much she missed doing this with him. Before Hogwarts they would stay in each other's rooms quite often. Whenever Regulus got like this Sirius would stay with her. He would lock the door so nobody came in, and he would just hold her until she could calm down. Even when she bit him bloody, he wouldn't let go. Still staring at her brother, she decided to unclench a bit. Regretting it when she remembered her clothes were stuck to her skin. Immediately scrunching her nose, she pulled at the robes desperately trying to get the wet spot off her chest.
"Can I help?" Sirius finally whispered. If Regulus was in a better state, she would think he was being cautious to not scare her. However, she wasn't really thinking all that much.
In fact, her only thought was, "Off."
It was a struggle. Sirius refused to just rip the stupid thing, but eventually it was off. The sighs of relief when it was over came from the soul. Now she was cold. Slamming her fists on the ground, she thought, Why can't anything go right, today?! Regulus had had enough. She was done with trying to be quiet. She wanted to scream. Where was her wand?
Realizing what she was looking for, Sirius spoke gently, "I've put a silencing charm on the door, Reggie. It's okay, you can be loud." With that, she started sobbing. It was like a dam had opened and all the pressure was released. Any thought left her head as she reached for her brother. Soon she was in his arms, no hesitation. Immediately shoving her face into his shoulder, she bit down hard and finally screamed. He only held on tighter.
It took hours. Hours of high pitched whining and sobbing. Hours of tight hugs and rocking back and forth. Hours of hair pulling and biting. Hours of sitting with her brother just like they used to do. It took hours, but she could finally breathe again. She gasped and it was like she had been drowning.
The first thing she said with her wrecked voice was, "I miss you."
There was silence and then an equally wrecked, "I'm sorry."
That was odd. Sirius hadn't been screaming, had he? Oh. He was crying. Sirius never cried. She looked up from where she was in his lap. Squirming around until she could grab his face, she looked directly in his eyes. She saw pain, he saw confusion. She saw regret, he saw realization. She knew he was never coming back, and he felt her complete devastation.
"I miss you," he says. A pause.
She echoes, "I'm sorry."