Hello my relatively small corner of fandom!
I have a few minor questions for you...
1. Why why why for the love of Mickey-God do you persist in thinking that Leon hates Yuffie? Whyyyyy? Because, quite frankly, he doesn't. They've lived together or very nearby in some way or another for nine or ten years, been working as a fighting team for awhile, enter tournaments together, are two of the heads of the Hollow Bastion Restoration Committee, and she can call him Squall without getting killed. All right, yeah, I can understand how FFVII-Yuffie and FFVIII-Squall could hate each other - because really, I can't see the two of them getting along well either - but fantards, you're forgetting one very important thing.
This isn't Final Fantasy.
I don't care what kind of justification you use for it, you cannot use their FF personalities as KH canon. I've only seen one person able to pull it off, and it was severely twisted and made my head spin. It worked, but it pretty much took too much thought to have it be a viable plan. So don't do it, okay?
Their lives are very different in KH, and I'll stop now before I turn this into another KH-isn't-FF rant. But yes, there is canon evidence that they get along and I can quote it from memory. After you fight Leon when you go to Traverse Town, you get one of two scenes depending on if you win or lose. If you lose, Yuffie appears and says "Hey, you found it! Good job, Leon!" and Leon goes "Still, it looks like things are worse than we thought. A lot worse." If you win, however, Yuffie appears and says "Aww, you're slipping, Leon."
And he doesn't get angry. Look at that again. She's teasing him, and if anyone can't take teasing well in these worlds, it's Leon (or Cloud, but he's not the subject here). In fact, he sallies back: "I went easy on him." Come on, if there was any point in any KH game that Leon would've blown a gasket, it was right then, after having been beaten by a fourteen-year-old shortie, with Yuffie making fun of him. Even when he does the "It's Leon" bit in the hotel later (after "I think you overdid it, Squall"), to me it sounds more like he's repeating something because it's tradition. He knows that telling her that will have absolutely no effect, but he makes the effort anyway. And it might just be me, but his "surliness" as the two of them are explaining everything to Sora could probably be explained by one simple fact: he didn't get the Keyblade.
After all, he's spent who knows how long since he met the King thinking about the Keyblade and that it can bring back the worlds and everything, and potentially everyone who was taken by the Heartless as well, depending on how you look at it. He's a trained warrior, a good one, with a lot of drive and determination - and he gets shown up by previously mentioned fourteen-year-old shortie with no formal training and very little stamina as far as he can tell. Who wouldn't be annoyed in those circumstances? So I'd hesitate long and hard (and look at his other scenes in all the games) before you say that his behavior then is normal. He doesn't hate Yuffie. He just doesn't.
2. WHY DO YOU MAKE YUFFIE SUICIDAL?! I'm a dedicated Squffie fan, but I frankly don't give a shit if they hook up if you make her want to KILL HERSELF OVER HIM! The girl lived through the DESTRUCTION OF HER WORLD and THE DEATHS OF HER FAMILY AND ALMOST ALL HER FRIENDS AT SEVEN YEARS OLD! She's survived and become one of the most powerful fighters in the worlds (because she is a ninja, and thus kickass)! SHE IS NOT GOING TO KILL HERSELF OVER ANY GUY! I don't care what sort of justifications you give it, it won't work! It won't, it won't, it won't! Because it completely ignores the inner strength and individuality that Yuffie has that makes her one of the greatest characters around. She's survived too much and gone through too many other things to just... give up over something so trivial. And yes, I said trivial. Romance is not the be all and end all of life, and plenty of people are perfectly happy single. I can easily see Yuffie as one of them. She does what she wants, and does not answer to anyone else - except potentially Cid and Aerith.
[/capslock of doom]
All right, I think I'm done >.>