I was going to say Toon Link, the weakest link (ha) of the three Links with nothing that notable or unique to the character mold... but the first two responses to this thread reminded me that Sheik existed, I probably wouldn't have remembered her otherwise if she hadn't been brought up.
I can't remember if I've ever fought a Sheik online. Maybe once, twice max? I can confidently say I've fought every other character in Ultimate at least enough times to not be able to count all the sets on a single hand. I recall playing as Sheik on a couple rare occasions when I'm screwing around with Random, but that's it. She has no real player base representation, the character herself was half of Zelda before the two were cut apart, with the latter growing into her own character and the former stayed mostly the same. I feel like if Zelda had been introduced after Melee, even in the Brawl era, I don't think Sheik would have been part of her character.
Sheik still being here after Melee is sort of a custom by now, ignoring Smash Ultimate's "Everyone is Here" mantra. I can see the justification for keeping her being her unique playstyle for a Zelda character in Smash, and the closest thing we have to someone playable outside of the Triforce Trio (she's Zelda, but not entirely, and certainly not a moveset clone of her, she has that going for her at least). On the other hand, all of her moves are pretty much original and draw barely any reference to her home game or series, and unlike Captain Falcon who does the same thing, isn't nearly as iconic, especially not now with, again, how underrepresented she is in high level play, or even mid level, I'd argue. Her popularity in past games mostly came from how good she was in the meta, but now that she's dropped off the map, she isn't seeing nearly as much love or attention. If we get a Smash 6, I feel like Sheik might be due for a replacement, a design overhaul, or just being cut altogether.
Thanks for coming to my TED talk about a shapeshifting ninja princess and why she doesn't belong in a mascot platform fighter series anymore