I can only speak for myself, but I'll try to explain everything the best that I can.
Can somebody explain to me why Ghirahim is seen as likely?
I don't really see him as likely in and of himself considering the idea of getting another Zelda character is just very strange to guess at because it recycles a small cast of people that we already have for the most part and rarely revisits anyone else that they've introduced as a major character. That makes the entire business of it very ambiguous. Not to mention Toon Zelda and Sheik/Tetra.
I understand why people like the character, and I like him too, but why is he considered the front runner Zelda newcomer?
Among the characters that are in the same position as him, as in, potential one-offs, we have characters like Midna and Majora-kid who've both been deconfirmed via assist trophies, and were both less praise (I think) and no longer relevant to what is being shown in Smash Bros. or modern relative to the audience (while Ghirahim and his game are the most relevant to Smash and to what has recently come out which Sakurai to a degree values), and even during the pre-Brawl days they battled with each other for a chance along with Wolf-Link and Zant, meaning among what was popular and relevant than wasn't very clear. Vaati and Tingle remain, and only the former is liked to a remarkable degree worldwide (as does Ghirahim) and has tons of potential (as does Ghirahim), while the latter is more of a Japan favorite (who is less liked than Ghirahim's similar-but-very-different flamboyant style; not to mention, many of Tingle's bonus points are from a spin-off which isn't related to the Zelda and thus a tad divorced from the Zelda representative issue). So it really comes down to whether Vaati's several appearances or Ghirahim's spotlight and relevancy (and what seems like more popularity) is worth more, and on a personal level I think the latter wins, and also from what we've seen with the Assist trophies and newcomers, I think his values might be a bit more aligned with the Demon Lord and not the Wind Mage. You've mentioned Impa, but I don't really see what's so interesting about her as a recurring character that has a small role, how she brings anything new to the table, or why Ghirahim wouldn't do a better job stepping in from Skyward Sword, being incredibly prominent in how it played out. And yeah, you can bring up Sheik as a potential detractor for her. So... Ghirahim is the front runner with all of this in mind,
to me. This is ignoring the confusion that is the Toon Zelda thing.
Not to mention the Ghirahim theme during the E3 Smashfest video and Fi trophy tease during the Smash-specific direct.
A character like him getting in to Smash Bros would be completely unprecedented. We've never had a one-off villain character become playable for any series.
Sure, but I don't see your point. We're not trying to say that he's a shoe-in, or at least I'm not. He's just the best choice out of what we've seen for this iteration.
Yes, we need more villains, but we need more villains from series that don't already have one. Most series don't even have their primary villains playable.
Well, Sakurai's lack of conern for what you think we need aside, you mustn't look at this like Ghirahim getting in means the rest of the roster is stagnant. Why couldn't we get some of those villains at the same time, as if his inclusion denies their possibility? It doesn't.
If Ghirahim got in, that would be like Fawful or Wart getting in for Mario, and both of those characters actually hold more relevance to their series than Ghirahim does to Zelda. Yes, Ghirahim is the spirit of Demise's sword, and Demise is the original Demon King, but this is a plot point constructed for one game in the series.
You made a claim and refuted it in the next sentence, regardless of how little you think it is. And it isn't, because it goes to explain something that never was before, something that apparently has been kept in a secret file for a while, something that apparently justified creating a timeline and creating the Hyrule Historia. So evidently Nintendo thinks something of it. The entire game of Skyward Sword offered explanations to things, and with it being most relevant on the home console series and with Smash Bros., having such an important game to the series AS A WHOLE isn't too out there and by virtue of itself sets Ghirahim apart from those you go onto list after this.
A lot of what you say here there's not really anything to say, so I'll be moving past it.
Hell, Skull Kid's actually been in three games, technically as many as Ganondorf, but that didn't seem to make a difference.
Yeah but we'd be playing as him only because he's being possessed with Majora, so really calling him Skull Kid is somewhat misleading and the other appearances don't really add to why anyone would want him in, even his Assist trophy has nothing to do with Skull Kid at all. Just a small point in passing here.
Ghirahim would make a great assist trophy, and that's really the best case scenario for him. He will absolutely be a collectable trophy though. So while Ghirahim has a chance to be in Smash, I truly believe that he will never ever be playable.
I agree that he'd make a great assist trophy and I'd be delighted to see that, but it's a bit much to assume that's the best case scenario. Without a doubt though, we'll be seeing a collectable trophy.
I'm honestly completely flabberghasted by why Ghirahim is considered likely. It's perfectly okay to like and support him, but if you honestly believe he will be playable, you're setting yourself up for disappointment.
Believing anyone will be playable, as far as newcomers goes, is setting oneself for disappointment. No one has even the slightest clue what they are talking about. Not even me, I'm just explaining why it seems from our limited perspective that Ghirahim seems like the most probable option if we are to receive one.