Disclaimer: JK Rowling owns Harry Potter; I just torture the characters.
Author's Note: This is mostly because Regulus never gets a humor fic. I don't honestly think JK Rowling or even Sirius's parents would be this unintentionally cruel. However, fanon has a penchant for giving the child, as a middle name, the Christian name of the corresponding parent, so when we found out Sirius's dad's name, this was far too easy and wouldn't get out of my head. And before anybody wonders, Arithmancy is basically divination using both numbers and names. Cheers! — Loki
Lily Evans stormed into the library, muttering angrily about Sirius Black and James Potter, the two biggest idiots she felt she had ever had the misfortune to lay eyes on. She made her way to the Herbology section— she had an essay due in two days she hadn't even started yet— but rather than pulling down a book, the fourteen-year-old girl stood by the shelf venting her temper.
"The disposition matches the hair, I see," a dry voice said behind her. "Not to say they don't deserve every bit of the tongue-lashing."
Lily turned to see a Slytherin boy sitting at the table behind her. He looked about a year younger, with black hair, grey eyes, and spectacles. The crooked grin he was wearing reminded Lily a little of Sirius Black. "What . . . what do you mean?" she asked, taken aback.
"About the red hair? Or about Sirius and Potter deserving it?" he asked.
"Black and Potter deserving it. Though, since you're Slytherin and younger, I s'pose they must have cursed you."
"Oh, quite often," the boy replied with a shrug. "You see, Sirius is my big brother. The name's Regulus." He held out a hand.
"Lily," she answered, taking it. She didn't want to ruin the friendly tone of the conversation, so since she was speaking to a Black she left her surname out.
"Well, Lily, I can't speak for Potter, but in Sirius's case he has to act like a bit of a prat. It's all in his name," Regulus told her. "Not that it amounts to a decent excuse."
Lily lifted an eyebrow. "What do you mean, 'all in his name?'"
Regulus grinned. "I'll show you." The third-year Slytherin pulled a scrap of parchment and a quill over to him. "All right, his first name is Sirius" —he wrote an 'S' on the paper— "and Mum gave him his middle name after our dad, and Dad's name's Orion." He added an 'O' beside the 'S' and his grin widened. "Of course, the family name's Black, so. . . ." He finished jotting down Sirius's initials with a flourish and a maniac grin to rival his big brother's. "Like I said, all in his name."
Lily stared at the scrap of parchment for a moment in silent shock, mouthing the letters. Her mouth twitched. "Oh, my God," she muttered.
"Exactly," said Regulus, now mock serious. "Sirius Orion Black, son of a bitch. And to imagine Mum said when I took Arithmancy that the subject was useless."
That was too much for Lily; she started to laugh. "Sirius Orion Black," she repeated. "That . . . that is amazing."
"Yeah, well, I've yet to explain James Potter this way," Regulus replied in the same deadpan voice. "Were I to ask Sirius if Potter's middle initial is 'A,' it would be a bit too much of a giveaway, after all. . . ."
Lily continued to giggle helplessly. Finally, she managed, "You put too much thought into all of this."
"Hardly. Mum screwed up so badly naming his you hardly have to look," Regulus argued. "And Potter acts like one." He shook his head. "Now back to real Arithmancy."
Lily looked down at the columns of letters and numbers beside this revelation. Many of them were crossed out, and it didn't seem as he had gotten very far. "Would you like some help?" she asked. "It's one of my best subjects."
Regulus hesitated— whether it was because she was a Gryffindor or because he knew she was Muggle-born, Lily couldn't tell. Finally, however, he nodded. "Yeah, probably."
Lily sat down beside him, flipped idly through the chapter for a moment, and then started to explain.
That was how Sirius and his friends found them an hour later. "What are you doing with my brother, Evans?" Sirius demanded. "Unless of course you're trying to convert him, in which case I wish you the best of luck."
"Convert me?" Regulus asked. He glanced from his brother to Lily for a moment. "You're a Mu . . . a Muggle-born?" he asked her. "Well, Evans isn't a pureblood name."
Lily nodded.
Regulus stared at her for a moment. Then he shook his head and muttered something about his mother.
Meanwhile, Sirius's eyes had fallen on the scrap of parchment lying beside Regulus's homework. "No, Reggie," he groaned. "You did not tell her my middle name, did you?"
"Hunh?" James asked. "What's so bad about your middle name?"
Remus must have likewise spotted the scrap, because he nudged James in the ribs, caught Peter's eye, and pointed to it. The other two boy's eyes widened as they silently made the connection.
Regulus looked innocently up at his brother, clearly fighting laughter. "But Sirius, it's just so . . . so fitting."
This was too much for Lily and the other three Marauders, and they and Regulus enjoyed another laugh at Sirius's expense.