Been thinking about Terra having a weird curious gay crush on Axel, who is is confusing for morality reasons ethics reasons gender reasons sexuality reasons, everything Terra built his self image off of Axel throws out the window. He's also really really annoying, but looking at Axel has the sane affect on Terra as looking at a goth girl on her way to the club making out with her girlfriend on the train would have on a 12 year old catholic girl. (Axel is more open about what he's done wrong with Terra since he feels he understands and he knows it's obvious he looks kind of evil and that doesn't matter to him if he does what he thinks is right. He knows Terra is a bit squeamish about darkness though so he tries to avoid him at first I think, but I think the more Terra learns about Axel and how despite his lapses into darkness he always pushes forward and tries to be kind instead of giving in, and I think he'd really admire that. Especially Namine's story.... Axel never thought of himself as damaged goods because he made a mistake. He dusted himself off, found something new to push him forward, and kept trying to be good, the pure essence of what "may your heart be your guiding key" was meant to mean. I think Terra would find that very very meaningful)
I got inspired by @peachy-ciel making an entire post explaining Vanitas/Leon to make an entire post about my personal favorite KH rarepair, Terra/Axel.
In essence, I think their personalities match up well and the two being unrelated to each other's past helps the two feel comfortable being open and fully honest around each other. (Well I mean, there's the whole Xemnas thing but for the purposes of this ship I tend to like to make this extremely minimal. Terra barely remembers anything, Axel fully understands that Terra is not Xemnas.) Plus, Axel isn't always the best person but he tries, and Terra's self perception is that he too, is very rough around the edges.
But you're not here for a brief rundown, you're here for the full deep dive on why Terra and Axel should kiss on the lips (with tongue). And I shall provide.
Axel's perspective
This all starts with Ventus, as a lot of things in Kingdom Hearts is prone to do.
Ventus is Axel's only living tie to his life before he became a Nobody, that's completely uncomplicated by what came after. And Axel deeply appreciates Ventus for exactly that. A time, a happy time before everything got so complicated, before Axel got himself tangled up in the Organization, and later Keyblades. Of course, now they're both involved in Keyblades, but the sentiment still stands.
One day, very soon after Axel and Ventus get reacquainted after the events of KH3, Ventus wants Axel to get to know Terra and Aqua better too. Axel agrees, though he's a little intimidated by the two. Axel so far has struggled to get a read on Terra, Terra being very withdrawn and keeping to himself. What he knows about Terra's past, especially the situation with Lingering Will doesn't help. And Aqua is generally perfectly friendly, but let's just say that Axel's gotten on her nerves before.
Whatever. They meet up, Axel is pleasantly surprised by Aqua's playfulness when she's in a casual situation and doesn't have to act as a Keyblade Master. No wonder Ventus and Aqua get along so well.
As for Terra... what Axel gets from him is something he never could've guessed. Terra was a bit standoffish before, something Axel later learns is from him grappling with his guilt and insecurity, but behind the shell is a very laid back but earnest person. ...Axel was honestly expecting someone much less emotional, as buff dudes he's known usually are, but hey that's why you shouldn't assume, huh?
So they spend more time together. Another thing about Terra that Axel finds out, is how tolerant he is of Axel's quirks. Axel will say silly things, or just go off on a tangent, and Terra doesn't mind listening. Sure, even he can't help but poke a bit of good natured fun at Axel, but Axel's very used to annoying people. Terra just doesn't get annoyed, or at least not nearly as quickly.
It's nice. It's really nice, actually, that someone responds to his deflective nature, not by forcing him to open up but not by letting Axel push him away either. And Terra seems like such a stable person in general, a very calm personality and very nonjudgmental, that Axel finds himself confessing more and more to him.
It's just comfortable, being around Terra. Oh, is this love? Axel won't lie, he always found Terra quite attractive from his physique alone, but now he's enamored with everything about Terra. Ah, he's falling hard.
Terra's perspective
Terra found it hard. Extremely hard to open up and get to know the other Guardians of Light. He knows that he looks like their worst enemies their worst nightmares. He can't change what he looks like, he can't change that he'll be walking around as a reminder of all their trauma for as long as he lives.
Any circumstance where he had to be around the others, usually as a part of being a Keyblade Wielder, he averted his eyes from the others and focused entirely on his duties. Aqua and Ven where the only people he felt comfortable around, but even then he couldn't help but feel guilty over how he had hurt them...
Then Ven brought Axel over. To get to know him, and Aqua. Well now Terra had to try and be friendly or else that would be extremely rude. And... he's heard things about Axel. He burned a man alive? That was back when he was a Nobody, sure, back when he was under Xemnas... but if nothing else, Axel would have to be hypocritical to hold anything Terra.
You know what, sure, he could give this an honest chance, just to make Ven happy.
Terra expected an awkward conversation about their past and just how much Terra reminds Axel of Xemnas, but to his surprise Axel just... wanted to make jokes? Play a game or two with him, like those frisbees he befriended Ventus with so long ago, kind of like Axel was... feeling him out as a potential friend.
Hey, this is more fun than he thought! By the end of the day he's constantly laughing, harder than he thought he was capable of anymore, and actually... looking forward to spending time with Axel again.
Fueled by the confidence boost that gave him, Terra eventually befriended the rest of the Guardians of Light, and even some others only tangentially related to their group. Terra's friend group didn't just consist of Aqua and Ventus, but so many others, and he had never felt like he belonged more.
But something about Axel made Terra want to get to know him better, spend time with him, more than anyone else he befriended. The others were great! But...
After a lifetime of being raised to follow the rules, to uphold order, it was a bit liberating, being around someone like Axel. Someone who wasn't so worried about that, someone to help Terra relax and not freak out as much about making a mistake or breaking a rule.
So Terra got closer to him, telling him more and more, without the worry that Axel would see him differently or worse, get scared of him in response. They're both rough around the edges, and Terra felt so comfortable around Axel, knowing he wasn't the odd one out with him. The tainted one. They were both tainted, in a way.
What more... Terra was getting super attached to Axel. Aqua would always be Terra's best friend, the girl he grew up with, the Wielder he deeply respects who he stands side by side with. Ventus too would always be his best friend, though even know Terra can't help but see him as the little brother he never had. Axel... Terra feels all hot around him, even when Axel isn't doing something funny with his fire magic.
This must be what they call getting a crush on someone. Liking someone. Well, that's something Terra didn't think he'd have to deal with.
Ok but please imagine the awkwardness of saix and xemnas having a relationship while axel and saix were broken up and the tension between them and terra when they all come back bc technically terra and saix were dating since he was xemnas and it’s a Very Weird Situation
More terraxel thoughts... consider... Axel likes watching the sun set, yeah? And sunsets are usually orange. Axel sees the orange and it reminds him of Terra because that's his color. In fact he invites Terra up to the clocktower one day to watch the sunset and points out that see? It's orange, just like that charm you always carry with you. And then Axel very unsmoothly transitions into asking Terra out.