Regarding Smash yes. Though, when not talking about the situation they are in referring to Smash, there are some ways Zant, Ghirahim and the Skull Kid/Majora's Mask can be compared. (their personality changes are among them) and I'm just counting those three antagonists there, not Midna/Wolf Link.
I'm counting one-off but viable and notable characters. They had slightly similar situations.
And yes, I'm only referring to Smash, as I was in the context of my original post, Ghirahim being a viable option for SSBFor. Mind you, he's not my pick, but I can't really see how he's an overall
bad choice. I think Tingle or Impa could bring more to the table as unique slots. Tingle for more crazy shenanigans. Having more than one of his own games and being notable among various games, and not just 2 max gives him some stuff to not just work with, but major notability. Impa, as noted, has 2 noteworthy games. Ghirahim gained some high popularity, if mostly due to excellent characterization, as well as could be easily a far more unique sword user than seen before.(and I can't believe people think him having a sword means Ganondorf couldn't.
)
IMO, for varying reasons, I'd say the most probable would be Tingle > Impa > Ghirahim.
Tingle's placement: His own games, does have a lot more popularity than people think, although the least from America, very important and key to beating more than one game, and takes a role in most games story-wise. Or outright necessary to beat the game. Only game I can think of where he has zero importance is Four Swords Adventure.
Impa's placement: Let's be honest, SS did help make her somewhat notable. She has some popularity here and there.(seems to be a vocal minority, though) Fairly recent(which doesn't mean much, but I can see her being looked at at least). She's played some decent roles among the Zelda games, although only four does she have a story role, and only 3 of those most players will see by default. She's in both combined games for the Oracle series, but only appears in one without combining them. A lot of her stuff is in the manual, though. Zelda 1 and Zelda II has her nowhere to be found in the game itself.
Ghirahim's placement: While he definitely is pretty popular, a lot find him creepy. He was also really well-designed and has great characterization. One of the few new Zelda characters that I think really shined in his first appearance. However, being that he's not part of the true main cast or directly connected to more than a final boss(who is a one-off, although clearly important), says he's not as important as he could be. I think he's clearly a viable choice.
And all of them can be very damn unique, so that respectfully applies. Keep in mind I may be wrong about their popularity status, so it's from my observations it's like that. It's an opinion on that front(so don't ask me to cite it, please. But if you could cite their currently world popularity, that's cool)
As for their roles outside of Smash, I don't find that much relevant. Impa barely shows up in an important role(3/4 games out of 6.). Tingle stars in 3 games, and he doesn't do anything important to the game in any possible way in only one game released. By important, I mean being useful in story or gameplay. Ghirahim very well was very important in his only appearance thusfar.(I forget his life status, but dead characters often get revived, so...)