I think Ralsei is Asriel

Cuz lightners turn into their undertale counterpart when they go in dark worlds (I think)

Undyne: she gets Armour and a spear

Toriel: Queen

Gerson: Great warrior and historian

And then Ralsei is a LONELY PRINCE like Asriel. Lmk if I'm missing someone

(BTW I know kris turns blue but like this only applies if they are the same person in UT/DT which we know from their relationship to the soul Kris/Frisk aren't)