Disclaimer: Of course I don't own Kingdom Hearts—that honor belongs to Squaresoft and Disney. I'm just playing in the sandbox.
Formatting: "Spoken words are in quotes." Internal character thoughts are in italics.
It's not like Axel feels things, really. He's a Nobody, after all, and no body means no thumping menace in the left center of your chest. But sometimes he and Roxas go out in search of Heartless, and somewhere in the middle of a charging horde of Darkness, illuminated by wild bursts of fire and the sharp flash of a key, there is this fierce, unrepentant rush of sensation that makes the synapses of Axel's brain light up as if his skin had suddenly become a sheet of licking flames. His thoughts are like meteors, bright with the friction of atmospheric entry, carving out feverish lines and arcs between the stars, trajectories of chakrams and blazing heat, his muscles quivering with barely contained fire. In those little half second lulls between the advance of one dark rank and the next, he'll sometimes catch Roxas' eyes—just the flash of irises, the color of blue sky, and there will be this little smirk, Axel's smirk, reflected on Roxas' lips.
They're not really friends, but there's just enough there to pretend they are. Maybe a facade is just as good as real to a Nobody—they don't have Hearts to tell the difference. So they go on their missions, go through the motions, swinging blades and sharing smirks and eating ice cream afterwards. They dabble in semantics, not minding that they never really talk about anything heartfelt. It would be rather pointless to try, given their nature, but there are idle ramblings and comfortable silences, mixed in with quicksilver exchanges of wit, and it's all too easy to forget the things that were never there. Sometimes, though, Roxas will say something, and—of course it's just pretend, but—he'll get the inflection just right, just bright or sharp or warm enough, and Axel will remember another time, another place, another life ago, when he'd searched and flailed and struggled on with what felt like a nova burning a hole in his chest, he'd just cared that much—
He hadn't been Axel then, of course, not in that Before time when he'd had a hope and purpose and a Heart. Now, he has his fire and his chakrams and a partner with a freakishly oversized key to help combat the mobs of Heartless that never seem to thin out. All the fighting would grow a bit tedious if it weren't the only sort of excitement that a Nobody really gets, and in the dashing, slashing, spinning fever of Keyblade edges and wheels of fire, there are these little reflexive recollections, these tiny shards of memory piercing into him at the sharpness of Roxas' smile...
It's not the same as having real emotions, nothing could be, but it reminds Axel of who he once was, how he once burned as bright as the stars themselves, and in the rush of heat laced memories, there is something stretching out in that empty place inside of him: echoes of his Heart.