Light is never sure whether she's a dream or not.
He'd always meant to kill her. He'd meant to kill her when she halved her life a second time, just for him. He'd meant to kill her when she wrote name after name, year after year, all for him. With every lie and every kiss, with every use he'd put her to, he'd dreamed of the moment he'd write her down. A page to herself, perhaps, after she'd served him so devotedly.
She'd been so grateful, so desperate to belong to someone, to be loved, and he'd made her one of his most useful weapons. Or rather, her eyes. Every word he'd told her about those had been the truth, after a fashion; was it his fault if she misheard? And then she'd given up her memories, and then Near had been dead, along with everyone who might oppose him, and he'd begun to reconstruct his perfect, fake life from its ashes only to find that he was bound to a zombie.
She didn't cling to him; he'd taught her better. She stared. She drank. The shattered remnants of her mind insisted she should be happy, and Light had made every effort to keep her out of his hair. He'd been doing it for years, after all. Just as soon as it wouldn't look suspicious, as soon as he was sure he could use her no more, he'd put an end to her suffering with a smile on his face.
Except that then she'd been dead - a silent suicide, done while Light ignored her for hours. He'd been genuinely shocked, and even affronted; how dare she kill herself before he could do it? And in his bath!
Except that then she'd been undead, and at his window every new moon.
"Light, my love, let me in, oh, let me in. I'm so cold." She pants all the filthy things she'd like to do, in that whisper that's too knowing to be a child's. Her face is white from the grave, and her eyes are slits; her lips are freshly clotted blood, and her teeth a mishmash of little glass needles. And who knows what she might be hiding under that torn white robe?
Isn't it perverse that he'd want her now, when she's dead? That part of him should want to get out of the bed - all his own now, the bed he shared with Misa - to throw the window wide, and invite her in? To have her knock him to the bed, all reaching, flaking talons; to feel those teeth - God, there must be hundreds - to feel them fasten in his throat, while she slides down him; to drain into her at each end as he dies. It's sick, to be so moved by thoughts of death and torment, to dangle his fingers over the meatgrinder just to know he can.
He lies there, painfully aroused, and listens to her lies. He's Kira, complete and entire, the unchallenged god of the new world, and yet even he can't kill what's already dead. By morning she'll be gone, back to wherever it is she goes, and he'll sit at his desk, puzzling out strategies, paring the world down to perfection, and not, definitely not, wishing he'd opened the window.