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Sayu knows she shouldn't be doing this.
She shouldn't be kneeling on the floor with her ear pressed to the wall that separates her room from Light's. She shouldn't be eavesdropping on her brother while he-
She can tell herself she doesn't know what Light is doing, and it would be the truth, she doesn't - not in detail. But it isn't the details that matter, because Sayu knows anyway that little sisters aren't ever supposed to listen to their big brothers making noises like that.
She hadn't meant to. It's almost midnight - on any other night she would've been long since asleep. But tonight she couldn't sleep, her body feeling sore and restless and her legs panging with the growing pains that seem to be happening to her all the time lately even though everyone at school says fourteen is too old for them, and she'd been reading under her blankets with a torch to distract herself from the aches with something nicer. Her precious copy of The Ruby and the Rose is well-thumbed, the spine cracked, the pages polished at the edges from her fingers on the paper, but even though she knows the plot by heart it still carries her away every time...
Or at least it had until she'd realised that in the quiet of the night, she could hear something from Light's room. It had sounded like a groan, and she'd been worried, that was all. Light has been so busy lately with exam prep and whatever it is he keeps locking himself in his room for, and while she knows she can't expect to be included in everything he does - he's older, he's a boy, brothers have mysteries of their own - she can at least care when he seems so preoccupied all the time. She'd put her ear to the wall just to check. In case he was feeling ill, or he was sad, or something.
It definitely wasn't either of those things. But by the time she'd realised that, her cheek against the cold plaster and the sounds of his voice carrying all too perfectly through the wall, it had been too late to not know what she's now trapped here listening to. She's sure her face must be red as flame. Her thighs are clamped together around the hand she's pressed between them, where she's cupping her fingers like she's hiding a guilty secret around the trembling, fluttering pulse of sensation in her-
Light - her brother - makes a low, broken, wanting noise on the other side of the wall, and Sayu bites her lip. She doesn't know what he's doing, doesn't dare think about it even if she was just looking at pretty boys doing it in the two-dimensional safety of a yaoi manga, but he sounds like it's making him feel good. A series of harsh, panting breaths, then another of those husky moans that's almost a whine, and Sayu swallows a whine of her own because that sound went right through her and left a coiling liquid heat in its wake. She should not be hearing this. Shouldn't be feeling this.
And then Light's incoherent noises become something more. "Mmmm," he purrs, hedonistic and wanton and Sayu doesn't know if she can blush any harder without catching on fire, and then: "Ryuk..."
She's never heard him say that name before. Sayu's breath catches, and her skin prickles and her stomach drops with a lurch of shock that's got far too many conflicting emotions rolled up in it. Who's Ryuk? A crush, after she's teased him so many times about never having any? Or - does Light have a boyfriend?
Before her thoughts can run any further, Light speaks again, and this time she's even more taken aback. "Yes, it's good," he murmurs, and his voice is loaded with a teasing huskiness she's never heard, one that makes him sound hardly like her brother at all. "Stop fishing for compliments-" and he stops and then laughs, pleasure-drunk and easy and heated, the pause between his responses just long enough for someone to have replied to him in the gap. "All right, all right... ahhh, yes, like that..."
Sayu's mind is racing even faster than her heart, and she bites her lip and presses her fingers hard against the ache between her legs because she can hardly breathe with what this is doing to her. There can't be someone else in the room with him, not least because she'd have heard the second voice if there was, but Light is definitely having an actual conversation with whoever Ryuk is. So he must be... on the phone? He has a cellphone, both of them do. Their dad sees the bills, of course, but Light could still get away with it if his - boyfriend? - was the one making and paying for the call...
Oh gods, she's listening in on her brother having phone sex with his boyfriend.
She wasn't ready for any part of this. And she curses herself for finding out this way, because with every fibre of her being she wants to poke Light in the ribs tomorrow and go "who's Ryuk?" and watch him fall over himself in shock before he tells her everything; and she can't, because now she can't possibly explain how she knows that name. If she'd only seen it on his phone or on a new year card or any other way she might have stumbled on it-
But no, she had to find out like this. Listening to her brother panting with pleasure on the other side of a wall at midnight, his ragged breaths coming quicker and his soft moans pitching shakily upwards as whatever he's doing feels better and better for him. "Ah - Ryuk, more... ohh-!"
Sayu stifles a whimper. She's caught in his voice like she's trapped in a net, she can't stop listening to this. She can't stop feeling-
Sayu has never, ever told anyone the whole truth of her feelings for Light. Her kind, handsome, brilliant older brother who's always been there for her, always looked after her, always been the central pillar of her life even more than their parents (because mum has so much to do and dad's job demands everything he has to give, but Light has always been there). She's never breathed a word that as she hit her teens, as her body began to change and she started to grow into herself as a woman, the sun-bright love she's always felt for her brother has evolved into something edged in fire. When her friends want to talk about crushes, she wheels out Hideki Ryuga and never acknowledges that a few more years and a few more inches of hair would make Light look shockingly like him. She never tells anyone that when she daydreams about her hypothetical first kiss, sometimes she's imagining her brother's gentle hands on her shoulders and his smile slanting across hers.
She doesn't feel as guilty about it as she thinks she ought to. Wanting to kiss your brother obviously isn't something you can talk about but she knows it's something people have done, and more so in the past. There are places in ancient history where the lines between brother and sister and husband and wife get hazy; she knows that, she's read about it. And she knows the story about how Izanami married her brother Izanagi and between them they became father and mother to a generation of gods and the whole of Japan itself, and thinking about that always makes something bright and brave and stubborn catch alight in Sayu's chest. Maybe that story means that being in love with your brother is more sacred than not.
Maybe there's something of the divine in what she feels for Light.
She even went to Imado shrine the day after her fourteenth birthday to offer a secret prayer to Izanami, asking not for Light to love her back - that would be far more than she thinks she really wants in the real world - but just for the goddess to see her heart and understand because it ached so much to keep her secret all to herself. She did feel better afterwards, and she still has the omamori charm she got that day on her dresser, but she's not sure even Izanami's mercy is going to be enough to help her now.
Not with this, listening to Light moan so shamelessly for a stranger who's sneaked into his life under Sayu's very nose and somehow got closer to him than anyone, even her. A helpless heat grows inside her, pooling downwards until it merges with the soft yet desperate ache held captive beneath her fingers. She's trying not to move her hand even though her body wants her to, afraid if she does she'll be the one crying out loud enough to hear through the wall. All she can manage is to kneel frozen where she is and keep listening.
And Light sounds - oh, he sounds like he's almost dying in pleasure now. Tight, high, desperate noises slip out of him, quick and ragged, and there's a very muffled rhythmic sound in the background that she thinks might be his body rocking against the bed. "Mmh... oh... oh I'm going to-!"
Sayu bites her lip so hard it hurts and swallows a desperate whimper of her own. Her insides clench as Light's voice breaks on a final high-pitched little cry of ecstasy, cut off almost at once as though he'd slapped his hand over his mouth when he realised he was making that perfect, damning noise. She buckles against the wall, shaking, listening to her brother panting for breath. "Ohhhh... that was good, mmm..."
And then he laughs, softly as though his mysterious Ryuk must have said something silly-sweet to him, and Sayu can't even be jealous because she loves her brother so much that she also loves anyone who can make him sound so happy. She wants to meet this Ryuk, whoever he is. Wants to give him a hug and thank him and also tell him she'll break his knees if he ever lets Light get hurt, because younger sisters aren't supposed to defend their brothers' honour but Light deserves to be defended and Sayu is willing to do it-
"Do whatever you like, I'm going to sleep," she hears Light murmur, sounding like he's halfway there already. "Goodnight, Ryuk."
She wishes she could hear whatever Ryuk says in reply, but after Light's words there's only silence on the other side of the wall. Sayu imagines her brother rolling over and settling into his bed, tucked up warm and satisfied and loved, and her eyes sting with overflowing affection at the thought. She loves him so much. She's happy for him. She really is.
Slowly, carefully, she gets to her feet and tiptoes back to her own bed, only realising as she creeps under the covers how cold she'd gotten kneeling on the floor. She shivers and bundles herself up, curling around the lingering fire in the pit of her stomach, her mind full of half-formed thoughts that are too complicated to do anything with right now. She really ought to go to sleep.
She'll decide tomorrow how she's going to find out everything there is to know about her brother's secret boyfriend. And then she'll be on their side no matter what.
Because Sayu loves Light, in every way there is, and that's all that matters.