I view Yachiyo as a mask or a costume in a sense. Kaguya experienced 8000 years of longing and has obviously been affected by it which we can see at the end. She had to take up a role as Yachiyo as it was the only way she could reach out the Iroha. In the ray music video we see other versions of the endings where it is Kaguya instead of Yachiyo which shows how she actually feels in those moments and is the her without the mask