It wasn't cheating. Kristoff hadn't cheated, he would never cheat. I mean, the last thing Maria had said to him - shouted at him, before slamming the front door behind her - had been we're through, it's over, and that was hardly ambiguous.

But he'd always thought that the definition of cheating was doing something you wouldn't want the other person to know about. And if they'd split up over Maria's belief that he was attracted to Anna, and then he'd immediately - that same evening - slept with Anna, that seemed like something Maria didn't need to know, once they were back together. So it felt like cheating, much as Anna shrugged it off.

They always did get back together, that was the thing, and he should have anticipated that and seen it as a relationship pause, not an ending.

"You think about things too much," Anna said. "What she doesn't know won't hurt her. It's not like we're going to do it again."

"Maybe I should just tell her."

"She gives me enough evils every time she sees me. Do you want to move out? Because you'll have to, if she knows we did it."

Anna flicked over the page of her magazine, the picture of nonchalance in her green sundress.

"Anyway, I thought we agreed we were never going to speak of it again."

"We're not."

"Stop speaking of it, then."

Stupid, stupid. I mean, what had she thought was going to happen? When she was being kind to herself, Anna would remember that she'd been operating in a normal universe with sane people where someone splitting up with their girlfriend meant that they'd, you know, split up with their girlfriend. But Kristoff and Maria seemed to be playing some kind of game where that was just a move on the board, and she had turned up at the door right on cue two days later, full or apologies and promises.

Including, apparently, that she was going to be nicer to Anna. And she was! In a way.

With the television turned up and the living room door shut, Anna could almost pretend that her flatmate and his girlfriend weren't having sex a few metres away. In the past they had tended to go to Maria's, and Anna wondered vaguely if this was for her benefit.

She stopped wondering when the living room door opened and Maria came through, wearing only the t-shirt that Kristoff had had on earlier. Yes, this was deliberate. Hooray.

"Hi, hun!" Maria said cheerily. "Do you have a first aid kit, or anything? Some plasters?"

"Um, sure. Just a sec."

Maria followed her through into the kitchen, and stood there watching while Anna rummaged under the sink. "Just one will do, Kris has like a scratch - on his back -" she looked at her nails smugly. "Make-up sex is the best, though, isn't it? Are you seeing anyone at the moment?"

"No. Not right now." Anna handed her the box of plasters. Maria peered at it. "These have Hello Kitty on them."

"They're the only ones I have, sorry."

"Oh, ok...oh, you know what! My cousin is single, I should set you guys up! We could all go on a double date, how about it?"

"Oh, I don't know…."

"It'll be fun!" She headed back towards the bedroom. "Well, thanks Anna! Talk to you later!"

Anna watched her go, gritting her teeth - but then smiled when she heard Kristoff say "It doesn't need a plaster, it's not even bleeding -" just before Maria shut the door behind her. Fine, he's yours, I get it. But he's a bit more mine that you realise.

It had been barely a month before doors were slamming again, although this time it was Kristoff, letting himself in at 11pm and shutting the front door hard behind him. Then there was silence, and Anna put her head out from the living room to see him standing in the hall, fists clenched and jaw set.

When she said his name he didn't turn. "I didn't think you'd be back tonight," she added, walking over to him. "Everything alright?"

He looked up, then, his expression bleak. "I just wish I knew what she wanted," he said. "I wish - I wish I knew how to be enough for her."

"Kristoff," she said hopelessly, and put her hand on his arm. He put his hand over hers for a moment. Then he sighed. "I'm going to bed. Goodnight."

Anna stepped back so he could go past her and into his bedroom. As he opened the door, she said "Kristoff?"

"Yes?"

"What is it you want?"

"What do you mean? Right now?"

"No, I mean..." What did she mean?

Kristoff leant in the doorframe and looked at her. She waited.

"I want," he said after a long minute, "to be able to spend an evening watching a film and cuddling on the sofa with my girlfriend without her breaking up with me because she'd rather go dancing. I want the woman who says she loves me to not describe me as 'dull as fuck'." He sighed. "I want to be someone's first choice," he said, then looked surprised at his own words.

The doorbell rang early the next evening, and Anna answered it.

"Sven! Hi!"

"Hi, Anna! Thought I'd come and take him off your hands for the evening. How's he doing?"

She let him in and shut the door.

"Not awful? But not great."

"In his room?"

"Yup."

"OK." Sven squared his shoulders. "Here goes."

He started whistling a jaunty tune and tapped on Kristoff's closed bedroom door. Anna laughed when she recognised the song, and sang along for one line - "how do you hold a moonbeam in your hand…"

Kristoff wrenched his door open. "I hate both of you," he said.

The problem, really, was the second time. Once, you could pass off as a mistake; as a These Things Happen; as curiosity satisfied, almost. Doing something a second time felt more deliberate.

Anna ended up going to the pub with Kristoff and Sven. Sven refused to let his friend remain maudlin, and said he needed the backup, then rang his fiancee and asked her to join them as well. Anna hadn't met Jessica before, but liked her very much, and the evening passed very pleasantly (except for a few awkward moments; such as when Sven said 'this is so much fun, you should bring Anna instead of what's-her-face every time, Kristoff').

They said goodbye at the pub door, and Anna and Kristoff wandered home. It was a clear night, warm and only just getting dark, and Anna realised that Kristoff was the most relaxed she'd seen him in a long time.

Afterwards she couldn't remember who had moved first. One minute they were taking off their shoes just inside the flat door, the next they were kissing; then she started giggling and couldn't stop, even as they were fumbling towards his bedroom, collapsing on top of the mattress together.

"We said we weren't going to do it again," she managed, when Kristoff pulled away from her to take off his t-shirt.

"Shush," he said, running his hands under her top. "We're not speaking of it."

She should find someone else. She should be a bit flirtier with the nice guy in office next door, she should go out more often, she should sign up with eHarmony, she should do something, anything other than sleep with her flatmate every time he broke up with his girlfriend. It was definitely a pattern now, and it kept happening, and it was bad for her. It was bad for him. It was...

...his mouth was on her neck, leaving soft kisses down and over her collarbone, as he slid a hand under the waistband of her underwear, as he groaned in that deep, urgent way that made her whole body shudder...

I don't want to be your comfort blanket, she thought. Not just that. But it had been months now, months of watching Kristoff have his heart broken by increments. And now they'd fallen into this pattern she had no idea how to end it.

She just wanted to be someone's first choice.