UNDERTALE Newcomer: Undyne
I know I'm not very active on this thread (and that's putting it VERY nicely), but I find the idea that Frisk and Sans are UT's only viable fighters kinda silly, so allow me to throw my hat in the ring with this pitch.
Out of all of UT's cast, Undyne is probably the one that'd be most likely to accept a Smash invite. Plus, she's a very popular character in her own right, with one of her battle themes getting remixed by ZUN!
As a fighter, I'd imagine she'd be very up-close and personal and wield a spear as her primary weapon, making her fit right at home alongside BWD. In addition, she could summon magic spears to give herself a nice projectile and suplex opponents, just because she can. Her brash, gung-ho personality would make for an interesting and fun translation to Smash, being a lot more damage-orientated than BWD at the cost of some mobility. If you wanted to, you could also translate her Undying form as a risk/reward transformation which amps up her damage/specials at the cost of taking constant chip damage due to the effects of DETERMINATION. Would be a lot more interesting than a standard comeback mechanic.
Overall, I think Undyne would be a fun add to the cast!