Likelihood
It's pretty obvious that Sora doesn't have the best chance of making it to the Brawl roster. With that said, I'm surprised that a more organized effort hasn't been mounted to point out that his chances even if small, are obviously not unlikely. In fact, I'd say he has a good fighting chance of making it and is the most likely individual to do so out of the entire Square-Enix character portfolio.
Factual Evidence
1. His eligibility has already been established since he appeared in a great selling GBA game (and it did sell well despite what many say; 1.5 million globally according to wikipedia) as a main character. The most common argument I see poised against this fact is that the game was a spin off and is, at best, a tenuous link to Nintendo systems. That simply isn't true. Chain of Memories is canon to the main KH storyline so it's a very significant game. Besides, Snake is definitely never reappearing on a Nintendo system and with the PS3-exclusive MGS4 upcoming, I don't see why Nintendo would harbor any special feelings against Sora especially since KH will likely make more appearances on Nintendo systems.
2. Sora was also a character revealed by Sakurai in the fan poll he revealed. This is solid confirmation that, at the very least, Sora has been considered. Sakurai did say that 1 or 2 more third party characters would be in Brawl but he obviously wasn't certain and at the time of the interview, the character roster was not yet finalized.
3. Yoko Shimomura, KH music writer, is also one of the hired musicians for Brawl. While she is a now a freelance composer, her most notable and critically acclaimed works come from the KH series. Further, her only experience in music for fighting games is Street Fighter II which sports a very different style than Smash music. As a result, it appears unlikely that she was hired to compose original works for the game especially since Nintendo has already revealed the title screen music in the first update and Shimomura is not involved. It only makes sense that the composers of the title screen would also handle the other original songs in the game since they will share the same musical style as in the previous Smash games. Also, every music update thus far is being handled by the original composer. I don't think Street Fighter II characters will make it since Capcom has plenty of other icons that should be prioritized. That leaves the KH soundtrack as her remaining big piece of work. In light of this all, I think it's very likely that Shimomura is going to create remixes of KH tunes.
4. In an interview, Nomura said it would be "interesting" if Sora was in Smash. I'm still trying to find the source but I'm certain I read it last year. Many have said that Nomura, being a fan of the cutting edge and Sony, would not allow his creations to appear on the Wii. That's not true at all. His most recent game, Subarashiki Kono Sekai (It's a Wonderful World), appeared on the DS. In an interview, Nomura said several KH projects are under development and that the next portable KH game would be either on the DS, PSP, or mobile phone. With the current global sales situation and especially the one in Japan, I'd be willing to bet my hat that KH will have a healthy, if not exclusive presence on Nintendo systems in the future.
5. http://gamers-creed.com/?p=407#more-407
Not the most reliable source but they have been spot on before. The next KH3 game is now rumored to be Wii exclusive. It'd make perfect sense for Nintendo and Square to announce both this game and Sora for Brawl at TGS next month. I suppose we'll just have to wait and see and hope.
Circumstantial Evidence
1. KH is the only Square Enix series to have an original composer on the Brawl boat. Looking at the trends of the music updates, I think that puts Sora well ahead of every other Square character (except for, perhaps, Geno) in terms of a likely Brawl appearance.
2. Sora is popular and KH has global appeal. I don't think any other Square character or series fits the bill except for Final Fantasy but with Dissidia upcoming and no composer, I don't think any significant Final Fantasy character will make an appearance in Brawl. I don't expect Dragon Quest characters to make it either because of the composer reason as well as the fact that DQ's appeal doesn't exist outside of Japan. The regular argument made for DQ is that a character appearance would skyrocket Japanese Brawl sales but that isn't true. The best selling home console DQ spin off is Dragon Quest Swords and that hasn't breached 500,000 yet. If Nintendo has a choice, the obvious choice is Sora since that would be the smart business decision and draw in a healthy-sized newfound global fanbase.
3. With all signs pointing to a massive Brawl roster, I'd expect Sora to have an even better chance of finding a way into the line up. After Snake and the obvious ones, I'd expect Sora to be next in line making him the likeliest unlikely character.
4. The recent Subspace Emissary Army update looks very KH inspired. I suppose this is a very specious link but I still sense some hints here.
Why I Think Sora Should be in Brawl
1. It's time for Nintendo to restore relations with Square Enix. Any fan of Nintendo must appreciate its return to the throne of sales. The other part of it is to recapture third party support. With Square making the DS its newest cash cow, I think Nintendo should extend its own resources and begin to forge even deeper relationships with Square. I'd love to see Nintendo back on top in proper form. What better way to start than adding a popular character of Japan's largest third party into one of its own popular games?
2. In my opinion, Shimomura is one of the most talented yet unrecognized composers of our time and KHII had one of the best soundtracks of all time. With Brawl being traditionally, a creative breeding ground for awesome music remixes, it'd be worth it to have even Sora's turd in the game just to hear some of her remixes for KH songs.
3. Sora presents some interesting gameplay variation to the Brawl games. He's a key wielder with projectiles and magic and would be easy to balance. The combination is very conducive to good imagination. Adding Sora would be a great way to keep things fresh and resist the cloning temptation.
A Moveset WARNING: Minor Spoilers
I'll update this if someone would like to make something more hearty but I'd imagine that Sora would be like Pokemon Trainer and cycle through his KH2 forms like how Pokemon Trainer cycles through Pokemon and sport a similar stamina system. Him wielding a Keyblade makes for some easy B and A attacks and he has some nice abilities that would allow his jumping and recovery to be quite believable and fitting. I'd imagine his Final Smash to be his Final Form.
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So let us discuss this. It'd be neat to have a nice conversation on possibly be the biggest unexpected addition to the game.
