Here is the updated cast:
Sans <-> Toriel
Asgore <-> Papyrus
Gaster <-> Asriel (Ralsei)
River Person <-> Chara
Muffet <-> Undyne
Grillby <-> Alphys
Mad Mew Mew <-> Mettaton
Snowman <-> Dummy
Asriel -> Monster kid
( I will answer the questions right away: formally, Asriel occupies two roles at once.)