Frisk (PDMG 173) 

TBH I'd actually prefer this to Sans getting in, I like Sans and all but this represents Undertale as a game better.

Ngl I really like this idea, been thinking all day about a Frisk Smash moveset funnily enough

My gimmick would be to have a neutral special based on the "FIGHT" command, complete with the timing based meter and everything. If you score 3 or more KOs with this, you become Genocide Frisk who is slower and more powerful, has different taunts, and sometimes acts without player inputs

I'm ngl this sounds potentially really unfun to play against. like, imagine your fighting a frisk and all they do is sit in shield and spam dodges all match. Overall just encourages overly-defensive play which smash games already have a overwhelming problem with.

To add to your idea, Frisk’s taunts will be super long, but doing them will exponentially fill up the pacifist meter, like if you were ACTing in Undertale