She always wore black. That was the first thing that Pippa Pentangle noticed about the mysterious girl at the back of the lecture theatre. It was a shame, she reflected halfway through a particularly boring potion theory lecture, because she would have suited other colours. With such pale skin and dark hair, she could have really pulled off a nice crimson. Or burgundy. Wine colours, that was it. The wine-coloured girl.

It was a ridiculous diversion and it almost cost Pippa dearly when the lecturer called her out for 'mooning at the window, rather than focusing on the board'. She didn't mind too much. Potion theory was not her strong suit at the best of times. It gave her something to look forward to rather than the lectures. There she goes, she'd think as the girl pulled her head down through crowds and marched forward, using her books as a shield. The girl like wine.

The second thing she noticed was her voice. It wasn't until November that she finally spoke, and even then it almost went unheard. She was so softly-spoken that only those around her even registered the noise, and it was their shock that alerted the rest of the class to the momentous event. Rows of girls swivelled in their seats to stare, Pippa mong them. Professor Moonshine was apparently so shocked she neglected to castigate them, instead fixing her gaze on the bright red girl at the back of the class.

'What was that, Miss…?"

"Hardbroom, Professor. Hecate Hardbroom. I said that the potion requires some additional eye of newt – three, I think – and then to be stirred counter-clockwise seven and a half times. At least, that's my understanding of the theory."

The Professor blinked, owlish in surprise, and moved her lips in silent calculations. Finally, as the rest of the class watched with bated breath she nodded slowly.

"Yes. Yes, I think that would do it. How do you have such a good grasp of potion theory already, Miss Hardbroom?"

The girl blushed, and the crimson of her cheeks sent a glow of triumph through Pippa. She was right. It was her colour.

"I've read a lot."

The Professor nodded slowly, and turned back to the board.

"Well keep reading, Miss Hardbroom. The rest of you, keep listening."

It took Pippa a few weeks to find the girl – Hecate – and finally talk to her. After every potion theory lecture Pippa would head straight for the back row to find her, and every time she'd already faded into thin air. Pippa tried sitting at the back, but still Hecate disappeared. It was impossible, and as the weeks went by Pippa became even more determined to find her. It was a challenge, and there was little she liked more.

Then one day, out of the blue, there she was. Curled into an armchair in a dusty corner of the library. Pippa almost walked past her, set on her own quest for an obscure text on chanting theory, when the flash of dark eyes registered. Spinning round, she gasped at the sight of Hecate, head buried in a book.

"You!" she said before she could stop herself.

Hecate looked up, a confused smirk playing around the corner of her mouth.

"Yes, me. You?"

Pippa felt herself blushing strongly enough to match her dress and waved a hand airily, attempting an air of nonchalance.

"Pippa. Pippa Pentangle. And you are…?"

"You seem to know who I am already. You might be the only person who does."

Pippa shook her head. "That's not true. You've made quite an impression. The mysterious girl with the brilliant mind."

Hecate's smirk twisted slightly and she frowned to match it, curling further into the chair.

"There are rarely mysterious people, just those people haven't bothered to talk to."

"Well here I am, bothering." Pippa said, crossing her arms and raising an eyebrow.

"Yes, you are. Why?"

"You seem interesting. I'd like to get to know you."

Hecate nodded, closing the book slowly.

"Hecate Hardbroom."

Pippa beamed and extended an arm.

"Lovely to meet you Hecate Hardbroom. I think we'll get along like a coven on fire."

It was spring before Pippa managed to really learn anything about Hecate. She was taciturn in the extreme, the polar opposite to Pippa. She wore her heart on her sleeve, but Hecate's seemed to be hidden deep underneath layers of black fabric. Bit by bit Pippa prised information out of her – her birthday (October), her favourite colour (red), her favourite subject (potions, unsurprisingly). Then, slowly, came the bigger things: she wanted to teach potions, to inspire students to love it at much as she did. She lived with a grandmother who insisted on propriety above everything. She'd been home-schooled by that same grandmother, and had fought over being allowed to attend teacher training college at all. She didn't have any friends other than Pippa. She was Pippa's friend.

It felt like a gift every time she told her something new. She hoarded the secrets and ran them through in her head at night when she couldn't sleep. It was perfectly normal, she told herself, to feel this way. Hecate was her friend. Her best friend.

In return she spilled out every secret she held for Hecate to hear. Her proud, proud parents and the pressure their pride placed on her. Her determination to colour everything she owned pink. Her problems with potions – 'I just can't get the theory right' – and her knack for chanting. And her greatest secret of all – her desperate desire to set up her own school one day.

"Think about it Hecate. You could teach potions, be my co-head. Make sure every witch can play to their strengths, no matter where they come from. Inspire a generation of potioneers!"

Hecate would watch her with a curious look in her eyes. Part excitement, part amusement – and then there was something else underneath Pippa couldn't quite place. She didn't push: she'd learned better.

It came to a head that summer, when they decided to enter the water-skiing doubles competition. Hecate turned up at the lake for their first practise wearing a long sleeved black swimming costume, which looked like it was at least a hundred years old. Pippa, who was wearing a neon pink one-piece ("Visibility!") couldn't help it.

"Why the sleeves?"

Immediately she cringed. She knew Hecate, knew better than to ask questions. If Hecate wanted her to know, she would tell her. That was how it worked between them, how it had always worked. She opened her mouth to apologise, but Hecate got there first.

"Let me show you."

Sitting down carefully on the grassy bank, Hecate began rolling one of the tight sleeves up until it reached her elbow. As she did, Pippa gasped. With every inch of skin revealed, she could suddenly see the beautiful patterns and symbols adorning Hecate's arms. Some were brightly coloured, some were stark and simplistic, some were lace-like in their delicacy. A few of them were even moving, gently swirling around her arms as she breathed. Reaching out, Pippa gently stroked one of them with a finger, following the spiralling lines of a pink rose as the petals slowly opened and closed. She heard a gasp and looked up, seeing Hecate staring at her wide-eyed. She pulled her finger back quickly, but kept her eyes locked on her friend's.

"They're beautiful. Where – I mean, why…"

"They're spells. I invented most of them."

Pippa blinked, then blinked again.

"Spells?"

"Yes."

"What do they do?"

Hecate tipped her head to the side and considered her friend.

"Well, different things. This one, " – and she pointed to the rose Pippa had touched – "is one of the simplest. It only opens and closes, but I find it helpful to look at sometimes. Calming."

Pippa nodded, sensing that she was on delicate ground. As Hecate's hand faltered over a stark black rune, she searched for a new question.

"Why hide them? They're wonderful, really they are. It must be advanced magic as well. Don't you want to show them off?"

Hecate shook her head quickly.

"My Grandmother would never forgive me. It started off as a minor rebellion, just practicing a spell I found in an old book, but once I'd started I couldn't stop. Now – I don't know. They're mine. I don't want to share them."

"Except with me." Pippa said, not thinking.

"Except with you." Her friend agreed, quietly.

In public the sleeves stayed up, but in private Hecate would roll them up to her elbow and share more stories with Pippa. There was the tattoo encircling her wrist in a thin band which resolved itself into numbers to tell the time when tapped; a compass which turned to face North (good for ingredient sourcing, apparently); lace patterns extending up towards her elbow which, according to Hecate, was the same design as the lace used in her mother's wedding dress; a tiny sun which rose in technicolour every morning and set in the evening to become a silvery moon; flowers which bloomed in time with her breathing, slowly furling themselves into buds and blossoming again a hundred times a day. Pippa could spend hours watching those flowers. The only one left unexplained was the dark rune hiding just underneath Hecate's wrist. Pippa did her best not to push – but realistically, that had never worked in her friendship with Hecate. It came to a head a week before the water-skiing competition, when the two of them were stretched out on the river bank in exhaustion.

Pippa flopped over to her side and reached out to stroke Hecate's arm.

"You've never told me what this one means."

Hecate shook her head.

"No, I haven't."

"Would you like to?"

"Well. You seem to know everything else about me."

"That's what I'm here for."

"Yes, apparently. That one is – personal. I don't mean – I will tell you, it's just – I suppose I don't really know what it means."

Pippa was not feeling any more enlightened, and her face reflected it.

Hecate huffed and sat up, stretching the arm out in front of her to stare intently at the rune.

"It's an old spell. Old old, runic old. I found it in a manuscript hidden inside one of my Grandmother's books and I translated it. Most of the manuscript was just standard fare - spells for healing in battle, compass spells, the like. Then there was this."

She tapped the rune thoughtfully.

"I'm not entirely sure what it means. From what I could translate, it said it was a spell to create a link between two people. Or reveal a link, the dictionary wasn't clear. The etymology behind the word is fascinating actually, it comes from – anyway. It wasn't clear. But I was interested, and I had a taste for marking spells by then, and I thought, why not?"

Pippa bit her lip, engrossed in the story.

"So you cast the spell, and the rune appeared on your arm – and then what?"

"And then – nothing. I've read up on it since then. A few other witches have managed to cast it and they had different runes appear, nothing like mine. There's some speculation that it might be a…", and suddenly Hecate blushed and mumbled the end of her sentence inaudibly.

"That it might be a what?" Pippa prompted.

"Well, some witches have theorised that the spell is intended to reveal connections between people. Potentially romantic connections. You could call the idea soulmates, although I've never really believed in the concept."

Pippa's heart skipped a beat. The words came out her mouth before she could register them.

"Cast it on me."

Hecate reared back, her face a picture of confusion.

"Why would you…what?"

Scrambling, Pippa sat upright and clutched her friend's arm.

"I'd like to have one as well. A rune. One of your tattoos. Please. Cast it on me."

Her heart sank as she realised Hecate was shaking her head. Turning, she squeezed her eyes tight to avoid tears spilling. It was ridiculous, but suddenly it seemed vital that she know, that she be marked too. A hand reached over to her shoulder, and she felt Hecate paused before resting it and squeezing.

"Not for why you think. You can't cast the spell on someone else, it has to be cast by the witch individually. I can – I mean, of course I'll tell you why, but why that one? Wouldn't you rather have a rose? Something bright pink, that I could do for you. That sounds more like you, Pipsqueak."

Pippa laughed at the nickname, but shook her head.

"Maybe after. The rose can be second."

Hecate nodded slowly.

"Come up to my room then and I'll dig it out for you."

The old vellum pages lay in front of her. She was sat cross legged on Hecate's bed, arm bared and shoes kicked off to rest disorderly by the door. In front of her sat Hecate, resting properly on her knees and looking worried.

"Are you sure you want to do this?"

Pippa nodded. She had to know. She had to.

Hecate glanced over to the door to check the silencing spells were in place, then turned back and fixed her gaze on her friend.

"All clear."

Taking a deep breath, Pippa began reciting the spell from Hecate's phonetic transcription. She almost stumbled a few times and every time she felt, rather than heard, Hecate hold her breath. Neither of them knew what would happen if she pronounced a rune wrong, after all. It struck her, as she reached the halfway point and felt sweat beading above her eyebrows that this might not have been her greatest idea. But no. She had to know.

Finally she reached the last syllable and closed her eyes, focusing on her arm. A stabbing pain struck her suddenly, making her gasp in pain. It moved a few times, shifting across her arm – and then, just as quickly, it was gone. She looked down gingerly, holding her breath.

And there it was. Her rune. Smaller than Hecate's – and for a second she felt a stab of disappointment. But then she looked closer. It may have been smaller, but in every other respect it looked the same. It was the same rune. Wide eyed, she looked up at her friend, whose eyes were filled with tears.

"Thank god." She said, simply.