1. His biggest fear is of losing Kai, in any way. Death, going home, their relationship falling apart for some reason, etc. He knows the latter is pretty unlikely, but he has doubts once in a while (and not nearly as often as he used to).
2. His gemini, if one ever formed, would be dominant, apathetic, scholarly and probably interested in books, not at all interested in food or sleeping, and entirely asexual.
3. In his almost two years in the dressing room, he has not once even thought of his Roxas. While they were both in the Organization, they were friends, but neither of them really knew what friends were, so at the time Axel thought they were best friends. Once Roxas left, and then Axel ended up elsewhere, he discovered that it was really just his FIRST friend, and that they hadn't really been very close at all.
4. He doesn't think he's anything special. As far as he's concerned, he's a twisted-sideways mess of a human being with a shitty past, whose only major talents are kicking ass, sucking cock, and showing up too late to do any good most of the time. He still can't fathom why he's lucky enough to have Kai, or anyone, with all that against him.
5. Previous point aside, Axel likes making people happy. Even if they're standoffish with him (see also: Girlyku), as long as there isn't open hostility, he's going to do everything in his power to try and get in their good books. If they accept his help (because that's usually where it happens; either because they're new to the DR, or they're wounded or something), he'll usually light up like a Christmas tree over it.
6. If you're at all close to Axel, you'll eventually be pretty close to Hachibi as well. If you're a stranger, or someone at all interesting, chances are good that if Axel's interacting with you, Hachibi is at the very least watching everything, if not making snarky or lewd remarks the whole time. It's not like he has a whole lot else to do inside Axel's head.
7. He can eat pretty much anything. Thanks to his bijuu's influence, he can easily digest raw meat and slightly spoiled food with no ill effects whatsoever, as well as being able to bite through and digest bone. He also eats roughly five times a day, and doesn't have as many nutritional needs as most humans.
8. As Marluxia and olderKai now know, Axel harbors a vicious, sadistic streak. It doesn't surface very often, except when he's angry or seeking revenge, and he hates when it DOES show up. That side of himself is something that came from his old life, the way he was raised, from Hachibi being encouraged to leak into his personality.
9. Axel's one of the few men in the world who's basically happy with the size and shape of his dick. It makes him happy, and it seems to make Kai pretty happy, so why would he want to change that? It's not like he'd be able to if he wanted, anyway.
10. Something even Kai doesn't know the full extent of: Axel's senses. He can identify people by scent, from a fair distance away, like several people in the DR, but it goes much further than that. He can track a scent more minutely than most humans could ever hope for; he can tell if a scent dead-ends into a wall, that it continues on the other side, and he's become pretty skilled at tracking a scent through a Nobody's portaling and the disruptive jumps in the trail that it causes. He can tell, offhand upon getting within six feet of someone, when they were last in a bathroom doing bathroom-y things, when they were last around food and what kind of food it was, what their basic mood is and if they're stressed/afraid/sad/angry, when they last bathed, when they last did anything sexual (within a couple of days; slightly less if they've bathed), if they've been to any of the outdoor rooms recently or around animals, and (with females) approximately when they're going to be bleeding next or when they recently finished. Considering his approach to dealing with people most of the time, you'd think this would lead to some major awkwardness, but it almost never comes up. He's so used to getting all that information in a split-second that it's not worth commenting on unless something is unusual about part of it. It would be like commenting on the exact shade of blue the sky is today, every single day, sometimes multiple times. No point.
11. Hachibi can't actually have bad intentions toward Kai. He and Axel both chalk it up to Kai's Light and pure heart, but it's still goddamn weird. He calms Hachibi down more than anything else in the world.