The fact of the matter is, Larxene remembers what it was like to have a heart. She just doesn't care. It's the thing that makes her stand out among the twelve: her lack of remorse and her lack of interest in her fellows. They, by contrast, are fascinated by her. The scientists pour over their new specimen, a cruel warrior and a woman at that. She is a scorpion that they will poke and prod with gloved hands until she raises her stinger to strike back. None of them want to get poisoned on account of their curiousity.
But Marluxia is different. She sees something of herself in him, in his confidence and his arrogant defiance that is always just shy of full-out mutiny. Kingdom Hearts never meant anything to him, it's just a game that Xehonart plays to occupy his eternity as a Nobody. And she knows that he does not play games; gambles, perhaps, as Luxord does, but he does nothing which offers no gain. Larxene can see quite plainly that he intends to gain something from her.
Their sparring matches are grisly. She admires the way he fights, with flower petals falling around him like rain, soft until the moment he wants to hurt her, when they become sharp as razors. They cut her skin; wounds that sting but heal before they can bleed, only to be ripped open a fraction of a second later by the bladewind of his scythe. For her part, she runs like mad, dodging him as best she can, snapping at his heels with lightning so that he can never stay in one spot for too long. Then she gets in close to slash and stab with her kunai, viciously attacking every inch of him he leaves unguarded. His eyes shine with bloodlust, and his breath is ragged like a beast's. She can't count the number of times they nearly killed each other like this, drunk on the fight, but she knows that she wouldn't be satisfied any other way.
When he comes to her speaking of alliences, she doesn't believe him at first. He says he's tired of the Organization's schemeing, but she knows there's more to it than this. She presses him, and he tells her he wants her to stand beside him; it's her loyalty, and no one else's, that he desires. She laughs at that. She asks him, If you had my loyalty, could you trust it? A Nobody can't care for anyone but itself, Marluxia, and you know that. He knows that she's right. He sheds his pretense, and when he says, "I don't want your loyalty; I want you." her body throbs with heat and she kisses him hard enough to bruise.
It feels good, it feels right, the two of them together like this, fighting even as they embrace. Marluxia digs his fingers into her skin and claws a trail down her ribs, leaving bright red marks and making her gasp in pain. In retaliation, she latches onto his nipple and bites hard. He reels backwards and she leans in, letting the sinewy contours of her body meld with the hard angles of his. They fit perfectly, like two puzzle peices lining up. She can feel him, hot and eager against her inner thigh. He begs her, "Let me come inside you. I need you." For a moment, she considers withdrawing; it's his voice, those desparate gasps. She wants to hear more, but her own need is too much. As much as he wants to be inside her, she wants him inside.
She winces when he enters her, bareing sharp white teeth. He urges her on, grabbing her breasts, her buttocks; fistfulls of her hair. His hands are everywhere and she arches up and moans. Their hips find an eager rhythm; she's slick wet inside and she feels Marluxia thrusting harder, more wildly. His mouth tries to form words, but only half-succeeds in yelping out between moans, "Oh!" "God yes!" and "Larxene!" before his head falls back against the sheets and he comes abrupt and hot inside her. She swears, for a moment, that she can feel his heartbeat.
She doesn't hold him long afterwards, because it doesn't take long for the heat to leave his body. When the sweat on his skin cools and he goes cold, it's like holding onto a corpse. She tells him to leave and he obeys, but not before getting down on his knees to lick the fluids trickling down her leg. She bends down and kisses him tenderly on the mouth, the taste of them both on his lips. "Go," she tells him, "you wanted me, and you have me. But I also have you."
To say that she loves him would be a lie; Nobodies are incapable of feeling such things. But she needs him, that much is true. He makes her feel more whole, more alive than she ever did when she still had a heart. To Larxene, he is worth far more than love.