"I'll send you the business proposal tonight," Regulus said, waving to Severus as he disappeared into the green flames of the Floo at the Leaky Cauldron.
Regulus let out a breath—his first time pitching a big plan to someone.
...
Flourish and Blotts.
Regulus picked up a copy of Curses and Counter-Curses by Professor Vindictus Viridian and casually dropped it into his shopping basket.
Sirius would probably like this one.
There was also a book on Transfiguration that mentioned Animagi—Sirius and the others hadn't yet thought of using Animagus forms to help Lupin, and Regulus didn't want to steal that credit from them. He'd give the book to Sirius and let their little group figure it out on their own.
"Hello, sir. I'm looking for The Invisible Book of Invisibility," Regulus called up to the manager, who was on a ladder organizing a shelf.
Like every wizard planning to do something under the cover of night, he urgently needed to study everything about invisibility.
The Disillusionment Charm was only the basics—he needed ways to avoid detection, and so on. He'd read a recommendation for this book in Transfiguration Today.
"Oh my…" the manager gave a helpless look. "Someone still wants that book? Listen, kid, we once stocked two hundred of them. They were the absolute worst—so expensive, and we never..."
"Well, we couldn't find them. We tried everything—Revelio, Summoning Charms… If you can find one, you can have it for free."
"I swear we're never stocking those again," the manager muttered, shaking his head and brushing off the unpleasant memory. "Never again…"
Regulus looked up at the towering bookshelves that stretched all the way to the high ceiling and couldn't help but rub his forehead.
Hmm.
It seemed this book really was impressively invisible...
Since he was already there, he wandered around the store and picked up several books at random, then activated his "sneaking" skill and began silently repeating The Invisible Book of Invisibility in his mind.
Maybe it was his sincere desire that triggered some kind of magic, or maybe it was the effect of "sneaking." Suddenly, he noticed the light on the bookshelf in front of him start to ripple slightly, like the effect at Number 12 Grimmauld Place.
A book squeezed itself out from between the others and appeared before his eyes.
The Invisible Book of Invisibility
He opened it. On the title page it read—
"Congratulations on finding this book. The first key to mastering invisibility is invisibility itself."
The manager nearby nearly fell off his ladder.
…
The book was practically a perfect manual on invisibility—
Making yourself invisible, making objects invisible, hiding traces of invisibility, detecting traces of invisibility...
It felt like training both a spear and a shield at the same time—and occasionally forcing them to clash just for practice. Or like training a programmer to launch cyberattacks while simultaneously improving their cybersecurity.
Every time Regulus finished a new chapter, the book would mysteriously vanish from his bedroom, forcing him to rack his brains and try every possible method to find it again.
And so began a new game between Sirius and Regulus: a shared game of hide-and-seek that included the book itself.
One day, they were both surprised to find the book sitting on Regulus's desk.
"Kreacher saw it under the bed and brought it up for master Regulus!" said Kreacher.
House-elf magic was truly incredible! They seemed to have a strange contract-like bond with the house, knowing everything that went on within it.
It was warm-up time for their daily dueling game.
"Kreacher's amazing, isn't he? Look at that magic—most adult wizards can't even manage that," Regulus said, casually casting, "Expelliarmus!"
"He's just a house-elf," Sirius replied indifferently, easily flicking his wand, "Protego!"
"I can't believe it—you sound exactly like Mum. Cold as a rock." Regulus didn't hold back. "Petrificus Totalus!"
"He doesn't even have his own will. He just blindly follows orders—" Sirius tried to argue, "Stupefy!"
"Who says he doesn't have his own will? He treats me better than he treats you," Regulus said, dodging nimbly and still having time to spread his hands. "Accio wand!"
"His dream is to chop off his own head and hang it on the wall—Depulso."
"That's just how he expresses love—Leviosa."
Sirius, defeated.
…
What confused Sirius even more was this—Regulus was always polite and reserved at school or around others, barely even spoke.
So why was it that every time it was just the two of them, he came out with so many sharp, soul-piercing remarks?
And why could he never win an argument against his younger brother?
Don't ask—whenever you did, Regulus would just quote some Muggle.
Stuff like "Books are the ladder of human progress," or "Everything we hear is an opinion, not a fact," or "Mind your own business and don't care what others think."
All said with perfect logic, sounding impressive even when Sirius didn't quite get it.
It seemed he might have to start reading those Muggle books himself.
But overall, this summer had been one of the best he'd ever had—
Ever since he'd overheard that conversation between Regulus and Snape in front of the family tapestry, he was certain Regulus wasn't like their parents—he was someone who thought the same way he did.
And with their parents' favorite, Regulus, covering for him, and Kreacher's favorite, Regulus, putting in a good word for him, Sirius found his life at home had become not just easier, but practically luxurious.
He hadn't even been punished once—he suspected the diamonds they gave their parents might've had something to do with that too.
Alright, he had to admit—his favorite person at home was also Regulus...
One day during the summer, something in Sirius's brain finally shifted. He admitted, at last, that Kreacher did have his merits and wasn't entirely rigid (especially when Kreacher helped Regulus conveniently overlook a few small things). Slytherin wasn't all that bad either. His prejudice against Slytherin no longer held up.
…
On the rooftop.
"You really are a true Slytherin," Sirius said with a hint of respect after hearing how Regulus had so expertly smoothed things over with their mother, getting her to agree to let him invite his Gryffindor friends over.
"What's wrong with Slytherin? Sure, I'm a Slytherin, but my name's Regulus—that belongs to Leo," Regulus declared shamelessly. "Even by Muggle astrology, my birthday falls under Leo."
"My Leo content is so high—if the Sorting Hat had put me in Gryffindor, wouldn't that have made perfect sense?"
???
Sirius stared at him, face full of question marks.
"Maybe I'm actually a descendant of Ravenclaw. There's evidence right here." Regulus said with a teasing grin as he used magic to groom the feathers of the eagles. "I consider myself loyal and honest too, kind and friendly—Hufflepuff probably wouldn't turn me down either—"
"So just tell me, which House wouldn't be reasonable for me to be in?"
I've never seen anyone so shameless in my life—Sirius couldn't help but burst out laughing.
The two brothers looked at each other, smiles growing in their eyes, and laughed out loud together.