Chara,who was once a human,was forced to kill everyone in Undertale.Sans tried many times to stop the cycle.then Frisk had appeared,the RESET button too.

Chara is half-human-half-ghost,given the fact that she supposedly died by the golden flowers when hanging out with Asriel in the past.

Whenever she appears,everyone either ignores her,or leave.Sans told them to be cautious of her,even if they don't remember anything after the RESET button was pressed by Frisk again and again. (You're Chara.)