He has wanted her since before he knew who she was, and after. When he knew she was his reason for coming to her world (the world that continues to be hers, even now she has her rightful place restored), when he didn't yet know what she was, he wanted her. Even let himself hope, snatching it in moments between the fighting and the nearly dying, that there was... could one day be, perhaps... something between them.
Hope is dangerous.
Hope is what she lives on, now.
And what there is between them, it's more than he could have imagined.
And now she's lying beneath him on the sun-beaten mossy grass of some nameless valley (not always - they are equals, because they can be nothing else - but yes, today, she is beneath him, and oh, the joy of it) and that same sun is merciless against his back: the rough cliffs towering above are enough to hide them from prying eyes above but little else, and they've long since left their clothing behind.
(They would never meet in the Whispering Woods, not for this.)
It is not true to say that no one else knows, though near as it can be: Spirit is as good as they are at keeping secrets. (And they are very, very good.) It is true to say that they're alone: Spirit understands this, too. In an hour, perhaps two, the beat of great wings will return and shatter him all over again, and he will have to clench his heart and hold this image of her inside him for another many months before he might get the chance to visit again, to see her again, like this.
(Months, or perhaps forever. This world with its secret places is a dangerous one, especially for her.)
But for now - for now she is lying beneath him, with moss in her tangled hair (at its shortest; because he loves Adora, not She-Ra, always Adora) and her blue eyes alive with heat, kiss-reddened lips licked wet and parted, soft gasps of breath at each thrust he makes inside her, and he feels more alive in this moment than he ever has with a sword in his hand.
Her skin is hot, hotter than the sun where her breasts press against his chest: her legs are strong around his hips, pushing him deep, and she never, once, looks away from his eyes. (Oh, the joy of it, that she knows all of him and still loves Adam, not He-Man, only Adam.)
They rock together, hands and mouths on skin; he slides fingers down between them, slick and rubbing, to make her moan. She digs one hand into the grass when she starts to come: fingers curve to the outline of a hilt she isn't holding, and her sharp, desperate cry has an echo of power in it that he can feel and then he's following her, as he always does, power calling to power in gasps and groans and half-breathed names, in rough hard thrusts and sharp scratching nails, until they're both bruised and sated with it, lost in it, lost to it. It takes them the way it always does, and there are lights behind his eyes if not in front of them, but at least now, here, they can fall into it together.
Afterward, he lets himself lie heavy on top of her. He wouldn't with anyone else, but she likes it, and he doesn't worry that his weight will crush her. Her fingers curl behind his neck; he can feel the sword-calluses against his skin. His own are rough on her thigh.
They can't be together, like this or not: that life was stolen from them and they can never get it back. She will always have been raised by a tyrant, and he will always feel guilty for having had everything that they should have shared. But he cannot, somehow, feel guilty for what they share now.
(Yes, she is his sister, but she was his before that, and he was hers.)