Asgore: Hufflepuff
Toriel: Also Hufflepuff
Frisk: Gryffindor
Chara: Slytherin (if you say it's the evil house, I may or may not stab you.)
Asriel: Hufflepuff
Undyne: Gryffindor
Papyrus: Tells people he's a Gryffindor, but he's actually a Hufflepuff.
Sans: Ravenclaw
Alphys: Ravenclaw
Mettaton: Slytherin
Napstablook: Hufflepuff