Step 1: Read Independence Day by Mystery Muse over on AFF. Step 2: Be confusingly fascinated by one throwaway line. Step 3: Write fic about it because it won't go away. Step 4: Forget fic for a month. Step 5: Trip over it again. Step 6: Post everywhere because why the hell not.
The first time it happened, he assumed it was a cooling malfunction.
Partners or no, X and Zero didn't get much of a chance to fight together very often; there wasn't much that would require both of them together, so they were usually split up, sent off on different missions. But this one had been-well, not difficult, just... insane: swarms upon swarms of tiny, insane mechaniloids, not enough to do much individually but actually fairly fearsome when put together. X and Zero had been sent in not so much because of their power-the new recruits could have taken them out if there were enough of them; it'd just take a while-but because of their speed. It wasn't a big threat, but if the horde got to a city it might have been, so there wasn't the time to chase them around.
They had just dispatched the last of them. X retracted his Buster, rubbing his eyes with his free hand before glancing up to see how his friend was doing.
For just a second, he forgot how to breathe.
Zero in battle was something terrifying and beautiful-feline and avian and serpentine all at once, a fluid savagery more dance than fight. X had known that before they'd even met. But to see Zero now, back to the blue reploid, silhouetted in red from the setting sun, his hair alight and flowing in the sunlight...
He was breathing hard, his sword arm slightly extended, the green flare of his sabre blinding on the shells of the fallen bots. His body was tensed, waiting for the next target, feet planted solidly on the ground, free hand open to grab the next thing that tried to attack him.
X had to cough and look away, heat rising sharply to his skin. Maybe the fight had taken more out of him than he thought. Maybe he was tired.
He'd feel better after he slept.
The second time, he knew something was up. Mostly because nothing was happening. Actually, literally, legitimately nothing whatsoever. X was curled up reading a book-an old one, made of actual squished trees-when Zero came into the room and flopped on the sofa next to him.
He looked exhausted. But not the kind of exhausted he usually looked, X noted with a faint tinge of hope. Zero didn't look weary, he just looked tired. And a lot smaller, his armour off, his hair unbound and getting everywhere, falling over his shoulders. X's gaze absently followed the wild locks across Zero's collarbone, down his torso, outlining the subtle muscle hidden beneath the basic black undersuit-
X dragged his eyes back to his book. "Do you want my recharger?" he offered.
Zero shook his head. "'m fine, thanks."
Ten minutes later, he was deep in hibernation, his head on X's knee. X found himself stroking the sleeping android's hair, watching him breathe, relaxed and trusting-nobody really slept anymore, not usually, but not even the smallest security system thrummed beneath Zero's skin.
Zero, of all people, felt safe enough to completely switch himself off.
The emotion, the thing, that blazed through X's heart then didn't have an entry in his database.
(He went to medical later that day. There wasn't a single thing wrong with him.)
The third time, it wasn't just him.
They'd been going... somewhere, doing... some...something. Maybe something important? X didn't know. Couldn't remember. Didn't really feel like it quite mattered anymore, because there he was, and there Zero was, and they might as well have been standing around in a vacuum for all X knew.
Except they were probably in a room somewhere, because he could see Zero pretty well and it wasn't the stabler-warmer outside kind of light, it was a quiet cold light that flickered a few times a millisecond and Zero still-
The blond seemed just as distracted, or entranced, or something, his eyes fixed on X's, guarded but wondering. X swallowed, very slowly-he wasn't human, he didn't have human instincts, but he did have a few... underlying protocols, a reploid equivalent, and one of them was waking up, spinning out through his circuitry and not making any damn sense.
"I think I want to kiss you," Zero said matter-of-factly, a faint frown starting to etch between his eyebrows.
The protocol sat up and started yelling at him.
"Why?" X asked a little breathlessly.
Zero blinked. "Because it's you?" The taller reploid straightened a little as if to back away. "If you don't want me to..."
The height difference was a problem, but X was just as strong as Zero no matter what Zero'd been built for. The brunet wasn't gentle with his hands, taking hold of Zero's helmet and dragging his head down, but he made up for the roughness by just, just barely letting their mouths touch.
Saying it felt like an electrical shock was actually quite accurate. Reploid skin-the originals' most of all-was an incredibly sensitive network of materials; the slightest touch sent their sensors into motion, analysing every conceivable detail from texture to temperature to radioactivity (which really, really tickled). And when that material met more of itself that wasn't part of its network...
It could get... pretty overwhelming, actually; not unbearable, but definitely to the point where most reploids covered all but their faces just so they could touch each other without getting bombarded (not to mention the other reploid's "skin" patiently trying to interface, linking their sensory platforms altogether).
X's mouth tingled. Alerts and analyses started pinging frantically through his system, his own diagnostics cheerfully reaching out to make sure Zero was okay, and with some effort he managed to mute them all.
X paused. He'd seen humans kissing, but... there was seeing something being done and then there was knowing how to do it yourself. He tried to search for a tutorial, but even the humans' network was superbly unhelpful. Go with your gut indeed.
Zero's lips moved awkwardly, the blond apparently finally having processed what was happening enough to bend down of his own accord, hands fumbling awkwardly for a good place to rest before ending up somewhere on X's shoulders. That's right, he barely even accepted hugs...
X's back met the wall and he stopped thinking.
Quick, heavy breathing, their cooling systems whirring audibly, the little soft sounds their mouths made against each other. X dropped Zero's helmet to the floor of his room with a clang, swallowed the Commander's protest with a clumsy kiss. Zero made a low, pleased sound, something very close to a growl, and X-wanted-wanted-something what was it
He had... parts, both of them did (Light wanted to make sure X could hide and Zero-well, who the hell knew), they could conceivably make some modifications, make the things more than vestigial, simulate human sex if they wanted to, but while that did sound terribly intriguing on the scientific level the-the Thing that every thought in his mind screamed for was something else, something different, across the species gap, and-and-
"It doesn't make any sense," he muttered as his breastplate fell and Zero's mouth descended on his neck, biting, licking, sucking, shockwaves and ripples of sensation pulsing through his synthetic skin.
"Mmh?" Zero obligingly replied, long fingers moving to the back of X's neck, taking the zip of his undersuit and tugging it down.
"We're androids, we reproduce asexually, we shouldn't-ngh-" (as Zero pulled the undersuit away from X's upper body and attacked a clavicle with his tongue) "-h-w-we shouldn't have romance-"
"Friendship is different-it's an evolutionary advantage on its own-but ohwhw-"
Zero's eyes were startlingly focussed as he pulled back, stilling the sensory onslaught. "Shut up."