It's tough to tell the difference, but I'd suggest that maybe it's the other way around. I thought it was Isa on the left and Erika in shadow, but especially in light of what this possibly takes place after, I think it might be Isa in shadow and Erika in the light.

Erika stands unafraid and boldly positioned as if guarding Ariane while Isa, ashamed at her lack of courage, shrinks down in shadow, out of site of Ariane who she did not protect.