Soh being Soh.

This gesture always amuses me. It reminds me of Soh's reaction to Yoshiro imitating her on the title screen.

Little details...

These things remind me of the sheer fabric on Soh and Yoshiro's outfits. Soh has 8 little flower flaps but Yoshiro only has 4.

The Cycle.

Yoshiro and Soh are one being forced into a cycle. 

Yoshiro is forever reborn to be the Maiden. 

Yoshiro holding a mask at the end, the new Maiden seen with a mask hints she's been reborn. 

I thought she became the new Soh but considering lore, they each play their roles forever.

Just a theory but this one seems more likely than Yoshiro and Soh switching roles.

Ancestors...

Is another theme of Kunitsu-Gami about healing wounds ancestors have left? Or fixing their sins? 

If we decide Soh was the previous Yoshiro and the current Yoshiro's ancestor, and when she's purged from defilement in the true ending she's able to purge current Yoshiro...

Extraction and the original 'Divine Maiden'

I suspect this is Soh being extracted from Yoshiro. I think they both descended as one and the ritual splits them into the two roles.

The split

The girl in the scrolls has two different faces: Soh's and Yoshiro's.

Just a theory. But the faces are definitely different yet close enough to appear identical at a glance.

The eyebrows. Soh is arched, darker and thinner. Her upper lip is thinner with a larger cupid's bow. Thinner nose. More upturned eyes. Less eye-make up. (iconic pink eyeshadow)

:3...

Soh has such a stern, cool and collected vibe. But underneath the mask is the sweetest face.

The two descended...

Both of them aren't human and both of them appear to be two halves of one whole. 

I'm wondering if they're the Goddess separated into two. 

But Yoshiro seems to be the main one, as Soh has no name and seems to be an extension of herself.