Disclaimer: I don't own Harry Potter, the Marauders, Regulus Black, Evan Rosier or Barty Crouch.

AN: This is written for hillstar's Character Appreciation Challenge using one of hillstar's prompts.

Chapter One: Waking Up

"Why would I want to talk to a Death Eater like you?"

Sirius's words echoed in Regulus's mind and he put his head in his hands. All through their childhood, Sirius had been Regulus's best friend – his 'go to' person when he was upset. But now that they were both older and in opposite houses, Sirius didn't want to know him anymore. Sirius didn't want to hear about his struggles with bullying, the peer pressure he was under, or the demands from his parents. No, Sirius was free with his Gryffindor friends and wanted nothing to do with his Death Eater brother.

Regulus crawled beneath the covers of his bed in the Slytherin fourth-years dorm. During the month after Sirius had left, his home life had been terrible due to the added pressure from his parents, and when he'd gone back to Hogwarts hoping for relief, the other Slytherins had treated him horribly. His family's reputation, which so far had kept the bullies from doing anything truly nasty, was in tatters due to Sirius's betrayal. So far, in the week he'd been back at Hogwarts he'd had his bag ripped open and his textbooks stolen, endured several tripping hexes on the Grand Staircase, had Mulciber mess up his potion on purpose with explosive ingredients, and had his head shoved down the toilet – twice. As well as this, he'd had to constantly listen to all of the Slytherins talk about what a Blood Traitor Sirius was, which had been horrible.

When he'd gone to talk to Sirius about it, to confide in his older brother like he'd always done before they'd gone to Hogwarts, Sirius had accused him of being just like the rest of his family. Sirius had said he was a Death Eater too, which was ridiculous: Regulus was only fourteen.

As he fell asleep, upset and angry with his brother, Regulus wished Sirius could understand exactly how he felt. Regulus knew that Sirius had been treated horribly by their parents, but now that he was free, Sirius didn't seem to understand what his disappearance had meant for his younger brother. Regulus wished that Sirius knew this so that he could help him.

Little did he know, something was about to change that would bring both Black brothers closer together.

…..

Sirius woke up feeling warm and wet. Normally when his friends woke him with water, it was cold and at his head end, not further down where this wetness was. He realised his had was in a bowl of warm water and immediately knew what prank had been used. He couldn't believe it: he'd seen this trick in one of the prank books he'd read as a child, but had always deemed it far too unpleasant to use on his brother, Regulus. Surely James wasn't that cruel.

He was about to yell for James when he realised his surroundings were completely wrong. Whereas his bed was normally surrounded by heavy, dark red curtains, this room was darker and his bed had dark green, silk curtains, held back with ribbon. The other beds all had closed curtains, but Sirius knew immediately that the occupants would not be his dorm mates.

It was obvious he was in a Slytherin dormitory. But why? And why had the other Slytherins pranked him? He knew this had to have been their doing.

He cleaned himself up and dried the bed with the simple household charms that his mother had forced him to learn. His mother's teachings were some of the only things he appreciated from his upbringing: if nothing else, Mrs Black had made sure her children attended Hogwarts knowing everything that would set them apart from the other students in their studies. He also cleaned and dried his pyjamas – since when did he wear green and black pyjamas? Swinging his legs over the side of the bed, he began to investigate the belongings of whomever the bed he was in belonged to. He wondered if it was Snivellus's and if he could find out anything from examining his trunk.

The initials on the trunk stopped him in his tracks. "R.A.B." He knew those initials – they were Regulus's. But what was he doing in Regulus's bed? Whose stupid idea was it to drag him into Regulus's bed in the middle of the night? Surely not Regulus's? Regulus hadn't put done anything out of line since the day he was born and Sirius was sure he wouldn't have had the first clue how to do a prank. Not sure what else to do, Sirius decided to open the trunk and find something to wear, or find clues as to what was going on. He wasn't sure which.

Regulus's trunk was extremely boring. It contained several sets of robes and uniform, folded meticulously neatly; plain underwear (with all the socks sorted into matching pairs), Regulus's Slytherin Quidditch kit and several books. There was also a small wash-kit and a white, fluffy towel.

Sirius disappeared off into the adjoining bathroom to get dressed. However he stopped when he saw his reflection in the mirror. For it wasn't his own. It was Regulus's.

…..

Regulus heard the sound of curtains being drawn back and felt the warm September sunlight on his cheek.

"Time to wake up, Sirius. You've slept in again," laughed a voice that Regulus vaguely recognised. He opened his eyes to see a black-haired bespectacled boy grinning down at him, who he immediately recognised to be James Potter, his brother's best friend and substitute brother.

He scowled. James Potter had been the one responsible for manipulating Sirius and turning him against Regulus. It wasn't that Sirius hadn't held the views he held about Muggles and Muggleborns before, because he had, it was that Potter had made Sirius hate Regulus. At first it had been letters home from Hogwarts that only talked about how amazing Potter was, then it had been Sirius ignoring Regulus during the summer in favour of writing letters to Potter, and finally it had been Sirius refusing to acknowledge him when he'd been sorted into Slytherin. After that, Sirius's ignoring of Regulus in favour of Potter had led to James becoming Sirius's brother instead of Regulus. It had all been Potter's fault and that was why Regulus hated him so much.

"There's no need to look at me like that, Sirius," said Potter, still laughing. "We're almost late for breakfast."

"I'm not Sirius," Regulus grunted, continuing to scowl at Potter. By now he could tell he was in Sirius's dormitory. It was just how Sirius had described it in his initial letters five years ago, which Regulus had still kept, and the two other boys he could see were Sirius's friends. He didn't know what he was doing in Sirius's dormitory – probably the result of some prank that Sirius had pulled – but he was too annoyed with Potter to really care.

"What?" asked Potter, his face briefly contorting into something resembling confusion. Then he burst out laughing, which only made Regulus more frustrated. "Don't be daft, Sirius," he said.

"Urgh! Can't you see I'm not Sirius," said Regulus. He made to stand up and stalk out the dormitory in frustration. Perhaps he could go all the way back down to the dungeons and change into his school robes before anyone noticed he'd gone: his life could only get worse if the other Slytherins noticed he'd been up in Gryffindor tower. However, as soon as he stood up, Regulus knew he'd made a terrible mistake. His pyjamas had disappeared and all he was wearing was a tiny pair of black shorts that barely covered anything.

He drew his arms around his body and stumbled back hurriedly, his cheeks hot with embarrassment. At some point in the night Potter must have stripped him. How had Sirius allowed that to happen? Of course he knew that Sirius favoured Potter over him, but surely Sirius would never have been complicit in kidnapping him and taking all of his clothes off? For surely Potter must have had to do that to get Regulus into his current situation. They weren't even his shorts.

Scrambling back under the covers, Regulus wondered where Sirius was. At home, Sirius had always slept in until he or Kreacher had woken him, so it was unlikely that Sirius had gone down to Breakfast. So where was he?

"What's the matter with you, Sirius?" asked Potter. Regulus ignored him, annoyed that Potter kept calling him Sirius, who he plainly wasn't. He knew he and Sirius looked alike, but even half-blind Potter, especially with his glasses on, should have been able to tell the difference between them.

"Are you ill?" asked one of the other boys. Lupin, Regulus thought.

"A little," Regulus said, hoping to get rid of the prying older boys. "Maybe you should go down without me."

The three older boys looked at Regulus questioningly, but went down to Breakfast without him anyway. He was glad to alone. At least he could get dressed without them staring at him. He rummaged in the trunk next to him for something to wear, hoping that Sirius had something that would fit. The trunk opened with ease, as it was so rammed full of clothes that it hadn't been shut properly, and inside it was a haphazard mess of clothing. Not wanting to wear any of Sirius's pants, Regulus slipped a pair of trousers over the top of the shorts he was wearing, then a shirt, tie, jumper and cloak. He was surprised that any of it fit, considering that Sirius was half-a-foot taller than him and actually had muscles. However, everything fit perfectly.

Next, to his dismay, Regulus learnt that Sirius didn't know how to pair socks or hadn't ever bothered to do so. He rummaged through the broken quills, Sneakerscope, Quidditch gloves, multiple pieces of scrunched up paper and underwear to find several socks, all of different sizes. He finally settled on two black socks that looked as if they may have come together in a multi-pack but that obviously weren't intended to be a pair. One had a green heel, while the other had a blue heel, but they'd have to do. Regulus found Sirius's shoes under the bed, put them on, and walked to the bathroom to have a quick wash.

The sight that greeted him in the mirror shocked him. At first, he'd thought it had been Sirius, standing behind him, but then he'd realised it was his own reflection. Somehow Regulus was now Sirius, or looked like him anyway.

…..

Sirius had come out of the bathroom, after staring incredulously at his reflection for several moments, to find that the other members of the dormitory were awake. He'd decided that since he looked like Regulus, he would pretend to be Regulus. He might even find out important information that he could use against the Slytherins later.

"Did you have a nice surprise this morning, Regulus," giggled Crouch, an unpleasant boy two years younger than him with straw-coloured hair. Sirius wondered why Regulus hung out with him at all: the other boy had a reputation for being callous and cruel, especially to those he thought weaker than him.

"Yes, your amateur prank was hilarious," said Sirius sarcastically, trying to sound haughty and as if he disapproved of pranking like Regulus did. He assumed Crouch was responsible for the bed-wetting prank, but he didn't know whether Crouch new about the body switching or not.

"What happened?" asked the blonde boy on the other bed, addressing Crouch.

"Regulus wet the bed again," Crouch said to his friend.

"Again?" the other boy asked, smirking. While the Slytherin seemed to find this amusing, Sirius was puzzled. Did Regulus really wet the bed? Surely not: maybe the boys just carried the prank out more often. At least he knew now that Crouch wasn't laughing at the fact that he and Regulus had switched bodies. Crouch simply thought he'd got one over on Regulus.

"Yes, Rosier, I'm starting to think little Regulus might need nappies," said Crouch. Rosier laughed at this too.

"I'm sure you can buy them in Hogsmeade, Regulus. It might help prevent future accidents," Rosier said, standing up and walking over to them.

"And if you kept your mouth shut, it might help prevent future accidents too," said Sirius, stepping closer to the other boy. Sirius noticed that with his now reduced height, Rosier was taller than him. So was Crouch.

"Ooh, has little Regulus grown a backbone. We can't have that can we?" Crouch cooed.

"Yes, something has to be done about that. Can't have him getting too cocky," said his accomplice. Before Sirius could find wherever Regulus had put his wand, which probably wouldn't have worked for him anyway, Rosier had whipped his own wand out. "Levicorpus," he said.

The spell Sirius had used so many times on Snape, and on Regulus come to think of it, had Sirius dangling by his ankles. Rosier levitated him upside-down and unable to fight back (which was extremely embarrassing, but at least no one knew who it really was), floating him back towards the bathrooms. When Sirius was hovering above the toilet, Rosier finally let him drop.

…..

AN: This story is turning out longer than I first thought, so I'm turning it into a chapter story. Thanks for reading and I hope you liked it. Next chapter, Regulus and Sirius go to breakfast as each other.