‘Just for a minute,’ came the muffled answer. ‘Literally a stopover. And then I’m going dancing. We’re going dancing.’

‘Lily. I have work tomorrow morning.’

‘I love you, Louisa. Did I tell you that? I really do love you. You’re the only one who …’

‘You can’t just collapse here like –’

‘Mmph … disco nap …’

She didn’t move.

I touched her shoulder. ‘Lily … Lily?’

She let out a small snore.

I sighed, waited a few minutes, then carefully removed her tatty pumps, and the contents of her pocket (cigarettes, mobile phone, a crumpled fiver), which I took into my room. I propped her on her side in the recovery position, and finally, wide awake at three a.m., knowing I would probably not sleep for fear she would choke, sat on the chair, to watch her.

Lily’s face was peaceful. The wary scowl and the manic, overeager smile had stilled into something unearthly and beautiful, her hair fanned around her shoulders. Maddening as her behaviour was, I couldn’t be angry. I kept recalling the hurt on her face that Sunday. Lily was my polar opposite. She didn’t nurse a hurt, or contain it. She lashed out, got drunk, did God-knew-what to try to forget. She was more like her father than I’d thought.

What would you have made of this, Will? I asked him silently.

But, just as I had struggled to help him, I didn’t know what to do for her. I didn’t know how to make it better.

I thought of my sister’s words: You won’t be able to cope, you know. And just for a few still, pre-dawn moments, I hated her for being right.

We developed a routine of sorts, in which Lily would turn up to see me every few days. I was never certain which Lily I would find at my door: manically cheerful Lily, demanding that we go out and eat at this restaurant or look at the totally gorgeous cat outside on the wall downstairs, or dance in the living room to some band she’d just discovered; or subdued, wary Lily, who would nod a silent greeting on her way in, then lie on my sofa and watch television. Sometimes she would ask random questions about Will – what programmes did he like? (He barely watched television; he preferred films.) Did he have a favourite fruit? (Seedless grapes. Red ones.) When was the last time I’d seen him laugh? (He didn’t laugh much. But his smile … I could picture it now, a rare flash of even white teeth, his eyes crinkling.) I was never sure whether she found my answers satisfactory.

And then, every ten days or so, there was drunk Lily, or worse (I was never sure), who would hammer on my door in the small hours, ignoring my protests about time and lost sleep, stumble past me with mascara-smudged cheeks and missing shoes and pass out on the little camp bed, refusing to wake when I left in the morning.

She seemed to have no hobbies, and few friends. She would talk to anyone in the street, asking favours with the unembarrassed insouciance of a feral kid. But she wouldn’t answer the phone at home and seemed to expect everyone she met to dislike her.

Given that most private schools had finished for the summer, I asked her where she was when she wasn’t at my flat or visiting her mother, and after a brief pause, she said, ‘Martin’s.’ When I asked if he was her boyfriend, she pulled the universal teenage face in response to an adult who had said something not just spectacularly stupid but revolting, too.

Sometimes she would be angry, at others rude. But I could never refuse her. Chaotic as her behaviour was, I got the feeling my flat was a safe haven. I found myself searching for clues: examining her phone for messages (pin-locked), her pockets for drugs (none, apart from that one joint) and once, ten minutes after she had come in, tear-stained and drunk, staring down at the car outside my block, its horn blaring intermittently for the best part of three-quarters of an hour. Eventually one of my neighbours went downstairs and thumped on the window so hard that the occupant had driven off.

‘You know, I’m not judging, but it’s not a good idea to get so drunk that you don’t know what you’re doing, Lily,’ I said one morning, as I made us both coffee. Lily spent so much time with me now that I’d had to adjust the way I lived: shopping for two, picking up mess that wasn’t my own, making twice the hot drinks, remembering to lock the bathroom door to avoid shrieks of Oh, my God. Gross!

‘You are totally judging. That’s exactly what “it’s not a good idea” means.’

‘I’m serious.’

‘Do I tell you how to live your life? Do I tell you that this flat is depressing, and you dress like someone who has lost the will to live, apart from when you’re being a gammy-legged porno pixie? Do I? Do I? No. I don’t say anything, so just leave me alone.’

I wanted to tell her then. I wanted to tell her what had happened to me nine years previously, on a night when I had drunk too much, and how my sister had led me home, shoeless and crying silently, in the early hours. But she would no doubt greet it with the same childish scorn with which she greeted most of my revelations, and it was a conversation I had only ever managed to have with one person. And he wasn’t here any more. ‘It’s also not fair to wake me up in the middle of the night. I have to get up early for work.’

‘So give me a key. Then I won’t wake you up, will I?’

She blasted me with that winning smile. It was rare and dazzling, and enough like Will’s that I found myself giving the key to her. Even as I handed it over, I knew what my sister would say.