A/N: Holy cow, it's not a Portal story! This idea was a few years in the making. I never really could buy that Midna's "true form" at the end of Twilight Princess was genuine. She seems far too familiar with her imp body for it to be new. So, for everyone who, like me, can't think of her as anything but a devious little imp, here's my personal pet theory on the truth of the matter.
She didn't expect to live. A soft, confident smile and safety were going to be her final gifts to him. His blue eyes and hope for a long life were the memories that would see her through the fight.
Except... Ganondorf hadn't killed her. Certainly he thought he had. He'd run off to challenge her divine beast with the cracked Fused Shadow in hand, leaving her buried in the ruins of Hyrule Castle. But fire and darkness aren't enough to kill a shadow and she drifted like smoke through the rubble, aching and weary. Link would end this, she knew it in her very core, because that's what the Hero of Time was for.
All she had left was the hardest decision to make.
It would be easy, so easy, to just let him go and think she had died. She could go home and bring out his memory to scour her raw heart and hope that he found happiness. He'd be sad, of course, but grief over the dead was easier to bear in her experience. But it wasn't fair and she was too weak to have her last memory of him be that terrified face, him certain he would never see her again.
"Nothing is ever fair," she whispered, drifting towards the field she had crossed so many times on a wolf's back.
There was no question that the mirror would be destroyed. It had nearly ripped apart two worlds and it was long past time that shadow and light parted ways. That was the only way to ensure that no more harm would come to either realm. It wasn't the solution she wanted, but any good monarch knew when her own needs must be sacrificed for the good of all.
Thus her decision was made for her. There was no way she could keep from breaking his heart, but maybe she might soften the blow. Perhaps if this parting betrayal came from a stranger he wouldn't hurt so much.
She wracked her mind for any memory of feminine beauty in the world of light. What had made his eyes light up (excluding all those recent times when it seemed to have been her)? In all the old frescoes and murals, what were the women like? It was all so different and alien to her, but she tried, working her tired magic to stretch out her little body, give it the illusion of grace and beauty. The final touches, ornamentation and grace, she took from what she'd seen of Zelda.
In the end, she felt defaced and stretched, but maybe Link wouldn't notice. Too tired even to drift she stumbled across the field on ungainly legs, trying not to trip on her robes. It wasn't long before she could walk no farther and simply sank down atop a hillock facing the sinking sun. Doubts crawled in her mind and she was on the verge of reconsidering when the light intensified around her.
Of course. The Guardians of Light would want him to be rewarded. She flinched under their brilliance, her crafted illusion flickering, but she had been touched too much by light to be pained by it. Footsteps and ragged breath sounded behind her and she steeled herself for a moment before rising to her feet to look into his face.
"What?" The look on his face would have made her laugh any other time. "Say something!"
Link's eyes were wide and a little lost, but the sound of her voice seemed to bring him to himself.
"Am I so beautiful that you've no words left?"
A giggle at her own joke stuck in her throat; personally, she thought this form was hideous. But Link didn't seem to care. His face split in a grin so wide she was sure it must hurt. He rushed forward, so glad to see her alive, but stopped before they might have embraced. Instead, he bowed at her faux regal airs, a gesture which hurt far more than she wanted to admit.
Deciding it was best to have done with it, like ripping a bandage off a wound, she whisked them to the mirror before the sun could sink much farther.
"Well. I guess this is farewell, huh?" She tried to be glib to stave off her own dismay. "Light and shadow can't mix, as we all know. But... Never forget that there's another world bound to this one."
Never forget me, she wanted to add but couldn't.
"Shadow and light are two sides of the same coin," Zelda began and she wondered if the princess of light knew what was coming. "One cannot exist without the other. I know now the reason why the goddesses left the Mirror of Twilight in this world. They left it because it was their design that we should meet." Any other day, she might have questioned Zelda's intelligence, but she kept the jab to herself this time. "Yes. That is what I believe."
"Zelda..." she wanted to yell at the woman, berate her for her naivety, but it would be a poor thanks for the help she'd given. "Your words are kind and your heart is true. If all in Hyrule are like you, then maybe you'll do all right. Thank you."
And then the final moment was there. She stepped toward the mirror on her awkward legs, activating the vortex.
"Well, the princess spoke truly: as long as that mirror's around, we could meet again..." She looked at them both, lingered on his face.
A tear, one of many she'd been holding back, slipped from her eye and she brushed it towards the mirror, her eyes never leaving his.
"Link...I..." The mirror cracked. "See you later..."
Quickly, before she could rethink it, she stood in the fading vortex, her body sifting into a thousand grains of light. Her eyes never left his face as it crumpled in dread realization. She didn't have a laugh to leave echoing in his ears this time, only a sad smile from a face that wasn't her own.
When she came to rest in the shadows of her own world the illusion was gone. She was her small self once more, naked in the soft twilight and bereft of her powers for the moment. Alone, she walked toward the sentry tower where the survivors of her people were waiting, the rest of her tears dammed up behind red eyes. She would mourn once her people were cared for.