Frisk: A toddler, young girl, 20 something woman 

Sans: Barney the Dinosaur, Patrick Star, Crit1kal 

 Alphys: Anime highschooler, internet nerd with a massive lisp 

 Mettaton: News reporter, gay sex hotline

Undyne: Jasper, Hulk Hogan, Kylee Henke 

 Toriel: Kylee Henke again 

Asgore: Prince Harry, every rich anime kid ever

Flowey: Small child on helium, Satan

Papyrus: SKELETOR. ITS ALWAYS SKELETOR