Su's room was on the second floor of her family's combined home and restaurant. This was sometimes convenient, as it meant she had easy access to the eclectic fusion of Chinese and French cuisines her father and his employees cooked each night, but at present it was a nuisance, as it meant she'd have to pass a heavy, creaky door if she wanted to sneak out instead of reading all her textbooks, as her parents insisted.

She sighed and looked at them doubtfully. The Standard Book of Spells looked the most interesting, being the closest to her idea of what Proper Magic entailed, but then again that would just make it tantalising because she wasn't allowed to use magic yet; and Fantastic Beasts and Where to Find Them would be interesting, but neither of her parents would accept it as being real study. She settled on Waffling's theory book and got three pages in before deciding she didn't fancy cross-referencing a dictionary every other page. With a sigh, she put it down on her bed and looked for a coin to toss between the History and Transfiguration texts.

"Here, use this," said the boy sitting beside her, handing her a used rail pass.

"Thanks," she said, and let it drop. It fluttered to the ground, face up, but she missed this, turning to stare at the boy. "Um. Are you from Hogwarts?"

"No," he said.

He was dressed like a Muggle boy her own age, in a smart polo shirt and trousers, but atop his scalp sat two long brown cat's ears. His regular human ears were absent. On longer inspection, Su noticed he had a cat's tail and slitted pupils, too, and his bare feet lacked the well-developed heel that humans need for plantigrade walking.

"You are magical, though," she said, not a question.

"Well, yeah." He stretched out behind her and opened the theory book where she'd left off. "Endogenous, adjective. Meaning coming from within an organism. So endogenous magic is the stuff that comes from in here" he clapped her thigh "instead of a potion or an enchantment on an object or something. I'd skip past the foreword, though, it's never important."

A number of questions flashed through Su's mind. After a moment, she settled on the first one she could think how to phrase without sounding rude, "What's your name?"

"Ephemeron," he said. "You can shorten that to Eff if you'd like." He looked up and gave a lopsided smile which didn't show his teeth. It was a very nice smile. "Curl up behind me; we can read it together, and we can help each other if parts of it don't make sense."

"Um," said Su. She wasn't completely stupid, she knew that one shouldn't immediately trust strange boys at the best of times, never mind cat boys who can apparently read one's mind. She hadn't told him which was the word she hadn't known. "I'm sorry, but why are you here?"

"It's a magic thing," he said. He brought his legs up to his chest; his tail wrapped around and settled on his knees. "Stop worrying. I can't hurt you."

"Can't?" she said.

To demonstrate, he leant forward and bit her thigh, hard. She shrieked, and it turned into giggles in a moment.

"Ack! That tickles. Why does it tickle?"

Instead of answering, he bit her again, and again, instead of sharp pain, tingles shot up and down her leg. Chuckling, she pushed him away; he rolled with it, then tackled her flat against her bed, biting her neck and tickling under her armpits. Now screaming with laughter, she brought her feet up to his chest and kicked out, sending him flying.

The door shot open and her mother looked in, concerned, wand in hand; Eff rolled under the bed. She took in Su's flushed face. "Su? Are you alright?"

"I – yeah, I was laughing," Su said, and then, with the instinct of every sensible girl talking to her mother, "there was a funny line in the book."

Her mother frowned. "In Magical Theory?"

"It was a typo, actually," said Su, inventing quickly, "that, um, what was it – 'As a young witch or wizard, you are hair to great powers'." Her mother raised an eyebrow. "Well, I thought it was funny."

"Don't study too hard," said her mother, stepping back out of the room. Su glared at the door as she closed it; just who had ordered her to read all her schoolbooks before term even started, anyway? But then Eff rolled out and lightly hopped back onto her bed.

"If you do that again, I'm turning you in," she whispered.

"Sorry," Eff said, his mischievous grin not slipping. "Are we going to read this or not?"

He lay against her, his head nestled under her chin, and lifted Magical Theory so she could read it; she reached over his body to steady the book. For a few minutes they read in silence punctuated only by his explanations of hard words and concepts.

"Eff," she said at length, "why is there a cat boy in my room, all of a sudden?"

"I guess you needed someone to talk to?" he said. His tail tickled her nose; she swatted it away.

"So you're here because I … magicked you here?" she asked.

Eff winced and put a hand to his stomach. "Can you … not do that?" he asked.

"Not what?"

"Ask about why I'm here," he said seriously. "Part of my magic is that it … hurts to talk about myself too much. Not exactly, but …"

"No, I see," Su said. Her mother had told her about magical oaths and a few similar enchantments; it sounded as though he were under one which forbade him from talking about it. It was probably the same thing that prevented him from hurting her. "What can you talk about, then?"

"The book, if you still care about that," he said. "Or your family. I don't know. Do you like Quidditch?" His ears twitched, and she reflexively reached up to scratch them.

They went through the books together over the following weeks, albeit glossing over much of the longer tomes like the Herbology and Potions texts, which were too encyclopedic to get into, and the Defence book, which despite being promising subject matter was quite dull. Eff really made History come alive, and they acted out some of the scenes, her in one of her mother's slinky black dresses (coming down past her feet) and he in a ridiculous tricorne he picked up somewhere; he cut slits for his ears. Su paid him back by making up stories about the Fantastic Beasts, which all involved brave heroines rescuing handsome boys from being trampled by rampaging monsters. They talked about which spells were likely to be important, either because they were likely to be on the curriculum or because they looked useful outside of classroom, such as the Unlocking Charm and some of the jinxes. They even cracked the Transfiguration book, although that was so intricately technical that even Eff couldn't figure out more than vague synopses of its opening paragraphs. In fact, there was only one thing he refused to touch.

"Chapter eight of Magical Theory?" Su asked. "I see you've even blacked out the chapter title from the contents page. I do hope you realise that my parents paid for this."

"Do me a favour and save it for school," he said. "The rest of the book doesn't depend on it, we'll read everything else, just promise me?"

"I guess," she said, making a note to go over it with a fine-toothed comb the moment she got on the train.

Every night, after she went down for dinner, he would vanish, only to reappear after she turned out the lights, when he would curl up at her feet and purr to sleep. This disturbed her the first time, but it soon became the new normal.

"How much of a cat are you, anyway?" she asked.

"Just enough to be adorable," he said, and butted at her hand, making her laugh and pat him. He had such fine hair, it was like touching velvet.

She had three elder brothers, of whom only the eldest, Nick, was also magical. He was her favourite, as the other two had grown distant when they found out that Su had powers and they didn't, but he'd moved out that year, leaving her with Paul and Matthew. When he graduated Hogwarts that spring, he found a job cultivating magical plants for a pureblooded family which frowned upon summer holidays for other people. This was great for him, she supposed, but had been lonely for her.

"Go down to the shop and buy us a few blocks of chocolate, would you?" asked Eff, leaping up four feet and deftly snatching Su's comb from Paul's hand.

"Huh?" said Paul, blinking. "Sure, mister, um …"

"Mister F," said Eff. "I'm Su's wizard tutor. And I bite. Grr. Don't tell your mother."

Su mutely accepted her comb and watched as her brother left her alone. "Can you teach me how to do that?"

"Not without your mother shouting at you," he said.

"She'd shout at you first," she replied.

"Ah, but she's not going to catch me, is she?"

Some days, when studying got to Su too much, he picked up on her mood and suggested they go out. She had to satisfy her parents by reciting potion recipes or talking about somatics, and as often as not they found fault and told her to keep studying, but on some lucky days she might persuade them that she'd been invited to play netball with some Muggle girls from school, and would then sneak off to a park with Eff. He diligently avoided her mother but Muggles were always vague and polite to him. True to his feline nature, he was uncannily good at climbing trees, and would acrobatically leap from one branch to the next, heedless of tears in his clothes (he seemed to have an inexhaustible supply of identical shirts, because any damage or dirt would mysteriously vanish each night). From the next branch up, he'd offer her a hand, and they'd climb to the dangerously slender upper branches and look out across town. On other days, she'd pretend to be a train or a heroic witch, and he'd laughingly act as her conductor or nemesis Dark Lord or a marauding dragon. Once, she knocked him into a duck pond; he retaliated by tickling her hard enough that if he'd been able to hurt her at all, she would've been hospitalised.

"I found these in your Mum's room," he said one day, dumping an armful of records and a player onto her dictionary, which was lying open again for some reason. She'd get in trouble if her parents found the records in her room; she made a note to smuggle them back later.

"Celestina Warbeck?" she said, making a face. "Urgh, Mum played this at my tenth birthday party, no wonder nobody showed up for the last one … does she seriously not have any Weird Sisters?"

"Grown-ups," Eff said, shaking his head. "The Furies have a new single out, Lockjaw, I think I can probably get a copy of that if I go into town."

"I've heard that on the Wireless," said Su. "It's wicked, right?"

"Yeah, I really want to see a live show sometime," he said.

"I heard they Transfigure a live cat into the organ on stage," she agreed. "When did you hear them? You've barely left my room since you got here."

"Rather you didn't ask," he said, rubbing his temple.

… … …

Before she knew it, she was at King's Cross Station, saying goodbye to her parents; as with her magical brother who'd graduated last year, they had elected to see her off behind the barrier in deference to her father.

"Mum, I'll be fine," she said yet again. "It's a train station, if I can pass primary school I can get on a train that's sitting right there – why would I want to bring a dictionary of all things – I do not love it – yes, of course I'll write you regularly – yes, Dad – if I don't go now, the best spots will be taken – yes, I'll be sure to make more friends than at primary – yes, goodbye."

And she took a hold of her trolley and pushed it through the barrier to Platform Nine and Three Quarters.

She'd got in early enough that people were still gathered outside the train, talking and buying magazines and lollies from a stand, but Eff had somehow still got there ahead of her. Even among other witches and wizards, he stood out; a pair of Indian twins and a girl with dull honey-coloured pigtails were behind him, staring and giggling. Feeling irrationally possessive, Su strode forward and hugged him.

"Are you coming to Hogwarts after all?" she asked. "I thought you weren't, but you're here, so …"

"No, I'm just seeing you off," he said. "I'm really going to miss this."

"What, being around other witches and wizards?" Su asked. "You never have before, other than me."

There was a beat.

"Oh," she said. "You mean …?"

"You might as well," he said. "If not now, it'll come up soon enough in class."

She rifled through her bags, pulled out Magical Theory, and flipped to Chapter Eight. Its title was Of intent and accidental magic. She put it right back down.

"Actually trying to do something like that on purpose with a spell is really, really hard," he said. "I mean, imbuing it with its own personality while having enough of your mind to know what you know without removing that from your own mind, and to have it be able to look things up and learn, without you even noticing … that's definitely not first-year stuff. It has to be accidental, right?"

He winced and held his left arm to his stomach, but kept his voice even. His right sleeve was empty, the fabric unravelling.

"But the flip side of that is that if it's accidental, then when you learn a little theory and become able to control your magic, when you realise what you're doing but not every last little detail of how you're doing it, then you can't do it any more. That's why I didn't tell you anything about myself or let you read the chapter; the more you understand, the less room there is for your intuition, and I … well, I guess I didn't want to go."

"You – this is silly," said Su. "Stop talking, you don't have to tell me this. I don't have to learn about magic yet; I can go to Hogwarts later, only after I learn how to control this."

Eff chuckled weakly and shook his head. His left hand fell apart into a cloud of sparkling motes of light, and holes began appearing across his shirt and trousers. "You have to forget instinctive childhood magic to learn the good stuff. And there's no way I'd let you miss out on Hogwarts just for a dream like me. I'm not real, Su, not like you are. Besides, you're a smart girl. You can't help thinking about it, and I wouldn't want you to, anyway."

She stepped forward and hugged him again. She'd lifted him up before while they were playing, briefly because he was about as heavy as she was, but now he was so much lighter, his body disintegrating by the moment.

"You're real," she told him. "I'm going to bring you back."

His laugh had turned into a cough. "I'm just a moment in time, Su. Don't waste your life."

"It won't be a waste," she murmured.

"Make friends and be happy." He could only whisper as his diaphragm gave out. "I had a lot of fun. Bye, Su."

She pulled him flush against her and kissed his lopsided smile, and a moment later he was shimmering dust in her hands.

She looked down as the particles slowly drifted off her robe and onto the platform and away. A moment later, the girl with pigtails walked up and tapped her shoulder.

"Is he – did he, um, Disapparate?" the girl asked.

"No," said Su, "he's dead."

"Dead!" repeated the other girl. "Is – d'you – was he your friend? D'you want to cry?"

Su raised her hands and blew gently, and the dust dispersed. She smiled. "After the summer we had? No, I don't."