Soh is a Maiden who's merged with the Goddess and becomes her Messenger by wearing the mask of the Goddess to allow the Goddess to act through her.
We play as the Goddess, subtly confirmed in an interview.
When Soh loses her health, she can perform the role of 'Soh' and of 'Yoshiro'(bestowing masks and guiding villagers) but she cannot purify. Purifying is linked to the Goddess.
Her spirit form resembles a tamagushi, a former yorishiro. We can still control this form because as the Goddess, we've merged with this Maiden.
Soh 'descends alongside Yoshiro' because she is the descended Goddess merged with a former Maiden(Yoshiro).
The merge appears to be complete when the Maiden is granted the mask. (normal ending)
In the prequel, the Messenger is described as 'nameless'. I took this to mean it was the original descended Goddess and not a Maiden-fused-with-Goddess.
But Yoshiro is 'nameless' herself as Yoshiro is just name given to every chosen Maiden. It's not a real name. Soh likely is as well, just a 'name' given to the Messenger, who is an ancestor of the Maiden.
The name was likely given by the Maiden.
The play uses 'Maiden' and 'Messenger' but no names.
A concise way of putting this all together is:
The Goddess is possessing a Maiden's spirit through the mask.
The Maiden, through prayer, merges with the Goddess. She becomes a Messenger, able to wear the mask of the Goddess to allow the Goddess to act through her.
Is the Soh in the play a previous Maiden?
It must be someone the Goddess is acting through.
Or is the implication in the normal ending that Yoshiro's spirit had been wearing the mask? But it can't be because this spirit ascends in the true ending.
