Most children outgrow their imaginary friends. Light Yagami never did.

He's never quite forgotten finding out he should have been a twin. Picture it: a seven-year-old boy in the hallway, engrossed in the secret game he's playing. His parents haven't yet made the connection that every year on his birthday they give him some expensive electronic toy, and Light then spends the day improvising with the box. The games only let you do what the people who made them could think of. The descriptor that skips through his mind, picked up from a newspaper book review, is Turing-complete. Something you could do anything you wanted with. Something you could twist into any kaleidoscope pattern you liked.

His mother, out of sight, but almost never as out of earshot as she wants to believe, is on the phone to her own mother, in Kyoto. They talk about uninteresting things. Irrelevancies, Light tells himself; it's a recent new word. Something that's beside the point. Something boring.

"I know. I know, mother," Sachiko is saying. "He'd have been seven today too."

Light's ears prick up. Another boy, born the same day as him?

"Having Light, it's like having two boys in one. He's a blessing, he really is. We're so lucky to have him." His mother sounds proud and sad and worried, all at the same time. The praise is typical, and for now, it still turns him warm all over. That sadness taints it, though; what has he done to make her sound like that? As if, he thinks, she doesn't quite believe what she's saying. But she must do. Anything else is inconceivable.

Sachiko is continuing. "But I can't help wondering what he would have been like. What it would have been like to have twins."

She falls silent, then, listening to whatever Light's grandmother is saying. But she's said plenty. The new facts don't collide in Light's head; instead, like snapping up a roller blind, they flash up a new picture. Twins. Born the same day as him, to his own mother? No, she's talking about him. About Light himself. He'd had a twin, and that twin had—

His little face pouts, in one of those taut grimaces children everywhere adopt when they refuse to cry. Crying's childish, and stupid, and he won't. Silently, he creeps up the carpeted stairs to his room, tucking himself safely out of the way. He badly needs to assimilate all of this.

The new information's knocked him beyond reeling. Two of me. There were meant to be two of me. Hidden from the door, he curls against the bedframe. Something is missing that he never knew was gone; his mother might as well have let slip that the wings that should have carried him were cut away at birth. That's why I'm so smart? Because I was meant to be two people, not just one?

Light knows, really, that it doesn't work that way, but the loss is still sickening, and beyond his understanding. He's still young enough to be unsettled by the way the other children can't keep up with him – by the way he's had to learn not to babble his excitable inner complexities faster than they can hear, or to show too often that he always knows the answer. It's something he's already intuited and made right, with his intrinsic, but ever more secretly slanted, grasp of people. Everyone else is stupid. I can be myself, and let them laugh at me, or – or

He's learned to be likeable – not charming yet, but friendly and sweet in the way small boys can be. He's taught himself how to outdo the rest without ever rubbing their faces in his superiority. But he still doesn't like it. It's built the wall between him and them higher yet. And now to find that there should have been someone else, someone who would have really been like him, and not stupid and childish-ignorant and shackling like the rest of them...

From that high, lonely point, perhaps it's natural that the mirror in the bathroom starts to catch his eye. It's right down in human psychology, he'll think, later on – that concept that reflective surfaces are windows to some other place. As if there was a soul that could be stolen. Not ourselves, but others. Not here, but somewhere else.

But when he's only just seven, and getting into the habit of smiling at the Light in the mirror, and seeing him smile back, or of putting his lips to the side of the frame and whispering his lonely secrets and complaints, he doesn't think of it in any such terms. It's just a game. It makes him feel better. It becomes another part of that complicated internal landscape that defines him.

The next year, he pre-empts the middle school curriculum by reading about the bombings of Nagasaki, where his father's mother lived during the war, and of Hiroshima. It's not a good plan. That night, he dreams of fire, and wind, and stifling heat; of the sky lit up bright enough to make his name a curse. For a week, he picks out only white clothes, so the flash he's waiting for won't burn him so badly. Standing inside the wardrobe, with the mirror door shielding him from the window and its bright sunshine, he imagines the other boy's face blistering and charring, and the scared brown eyes running like wax.

The year after, he's nine, and one spring weekend Sayu brings him a tennis ball in the park. He adopts it; after a quick negotiation with his mother, the ball goes everywhere with him. It's always in his hand, poised in the tips of his fingers. He bounces it off everything – or, at least, everything outdoors, or out of an indoor adult's awareness.

When he's thirteen, he wins a national tournament for the first time, and long habit drives him to the mirror in his room, to bounce on his toes before it, and to see what his twin thinks. There's a raw joy in the other boy's face, unrefined and sweet like sugar. But when he's fourteen, he hugs his knees and rests one hand on the frame, and externalises all the things he's not allowed to feel – betrayal and shame, disappointment and rage. He puts them all onto the boy behind the glass. It's that other boy who sobs, so that Light never has to. Far better to comfort his twin than to think about what a disappointment he is to his parents, or how, yet again, his habit of eavesdropping has twisted back and savaged him. As if he has the right to waste his talents banging a ball around. As if it was given to him to do anything so pointless. Hasn't he known all his life what he's supposed to do?

The ambition's been there so long that it feels like his own. He's patterned himself after his father without ever realising it. Resettling onto his old path, he excels at it easily, just as ever. It can't be any other way.

And then, one day, it's 2003, and he's seventeen, and set on a path with no turns.

It's late that December night, hours after L confronted him, hours after Light fell into that neatly-set trap. He feels so strange. Things aren't as lurid and bright as they've been, the last few days. Something that was new in him has shattered into pieces: a rapidly rising madness that he's left behind, except – it doesn't feel like a good thing. Light's lost something, and feels like he barely had a chance to learn about it.

"Ryuk, take a walk. Or fly, or whatever it is you do." It's just the same bored authority he's used all along, with the shinigami – who complies; it won't always be so easy, but just now he's still sounding out this odd, human creature.

Moving to the wardrobe, Light's face brushes the mirror's frame. One hand presses against the slats of the door. It smells of pine, heady and fragrant. His twin looks back at him, one huge brown eye peeking around the corner, nestled in pale hair. He watches Light with his usual infinite understanding. But there's something else there too, this evening - furious determination, and leftover anger, and fear – the reality, unavoidable now, that there are going to be people who won't see the righteousness of his cause. What you're doing is evil. Or, as Light had heard it, with his signature inability to separate people from their acts, You're evil. But the boy in the mirror understands. He always understands Light. He's the only one with a hope of it, because he's the only other one that's really real.

Light wants to touch the glass so badly, but he never does. Come out here; you ought to be with me. Instead, one hand slides up – oddly, he usually moves up to caress himself at moments like this, not down – and long fingers trail over his twin's face, and tangle in his hair with soft tugs. Always, they mirror each other.

He's never named his twin; he's never felt the need, and it's not as if he could ask his mother. And yet – unpleasant as it is, as much as it misses the point of his plan, hasn't Light been given a name for him now? For this shadow who's trailed him his whole life long?

"Kira," he murmurs – even though sound doesn't carry well from his room, he's always dropped his voice for his mirror-twin. He's so close by, after all. He knows just how stupid he sounds, and also how perfectly the question fulfils his oldest remaining fantasy. "You're Kira, aren't you?"

There's an unpleasant quality to the other boy's silent smile. Resting both hands anew on the door's slats, Light looks closer (the glint of the eyes, the angle of the mouth, all the suggestions that his reflection knows secrets he couldn't ever possess), and closer, and closer yet...

His hips bump against the glass, along with the tip of his long nose.

There's a moan – a tiny thing, unsurprised, barely there at all. He can't leave marks behind. Oh, they'd wipe off well enough, but it's a matter of pride to him that nobody could ever tell how much time he spends here, contemplating himself. It's not like love, he knows. It's just another of the odd intrigues that entertains him. He'd never think of it as childish. Watching the reflection, and its eyes that track his own, and resting in its gaze, he notices an intrusive, adolescent pressure. His twin looks momentarily pained – then that smile becomes a little more knowing, a little more wicked and – what is that, inviting? Is he flirting with himself? Really?

Does it matter?

The doubt's gone in as much time as it takes for him to grab the tissue box from the shelf behind his desk. As he retakes his position, hunched against the door, he closes his eyes to see himself better. The frame is cool, woodgrain and varnish beneath his touch. He can smell the glass itself, an icy note where his breath mists against it, coming out in quiet starts. One practised hand slides down to unzip himself, to wrap around all that loose, sliding skin.

Light opens his eyes, peeking out from beneath his fringe. The curled-under smirk of the boy in the mirror is as hard as the glass he's trapped behind – or as hard as the cock in his hand. The slow, sticky, gliding sounds are lost in his rising excitement.

This isn't something he usually enjoys. It's more of a necessity – embarrassing, something he's only come to in the last couple of years, and accidentally, at that. But today, it doesn't feel as if it matters. That was the point, wasn't it? He can do anything he wants to. Anything at all. I'll show you, L. You can't stop me. You didn't embarrass me. You don't have a prayer of catching me.

Behind the glass, the other boy's face is flushed and almost feral.

Closing his eyes again, Light shifts the angle of his fingers expertly. His mind drifts to places that surprise him – the other boy's ghost hand coming out of the mirror, cold against his hot, swollen skin, touching him, helping him. His grip loosens a fraction, to give himself more room. The sounds he needs to make are getting harder to repress – but he still hears them, embarrassingly incoherent, aggressive and needy all at once, an inarticulate voicing of what he'd never word as keep going, don't stop, don't you dare stop.

With the rhythm of a metronome, the sounds chime in his head, and stifle in his throat. Ah – ah – uh..!

Bracing himself against the door's edge, he fucks the glass, pressing himself against it. He can see the smudges so clearly in his mind's eye, all the greasy giveaway marks he's sticking to this window on himself, but he can't find it in himself to care, any longer. The chilly glass is a shock; it's cold water down his neck, or long, strong fingers pressing an ice cube against some exquisitely sensitive spot. And finally a sound escapes – far too high, twisted and squeaking, strained and disbelieving, not at anything he's doing, or anything he's done, but at just how good he can make himself feel.

Arching his back, thrusting frantically into his hand, he shoots all over the mirror, sticky white trails that cling to the glass he's never touched before. He falls to his knees, shocked and spent, eyes glazed; someone might as well have grabbed his shoulders and pushed him down. His twin is panting, lips pink and parted, eyes febrile and dazed, staring into Light's own eyes as if he could eat him - and his face, too, is glazed behind the glass, white rivulets creeping downwards. Against the mirror's chill, they gel and slow down. Light is riveted. Darting his tongue out, catlike, he tastes the traces of semen on his hand. It's sour, unfamiliar, but not unbearable, and, and... leaning forward, he dabs at the cooling slime on the mirror, tongue against tongue, lapping at his own face. The texture is tacky.

As his breath finally settles to something resembling its usual self, his twin's face creeps back into something rather more quietly amused. Why did he do that? Does it matter? Who is there to stop him, any longer? Who is there to dictate what he should and shouldn't do, except himself?

Before he looks back, silent black wings ghost back through the wall, indescribably entertained by what they've seen.

Ryuk lets him take his shower without too much interference. The apple helps. And later on, Light takes the window cleaner from his mother's cupboard, and disappears the evidence into a disposable cloth. And as he crouches there, wiping up spit and semen – "It's perfectly normal, Ryuk. Just something humans do from time to time. Didn't you realise what you were getting yourself into?" – the god of the new world smiles just for him, bright and secret, cold and alone and there forever, his perfect, untouchable equal painted in silvered glass.