I think yokai are immortals even when their killed they will be reincarnated again and again forever, while sei probably met Kuchisake-onna and fell inlove until the present Kuchisake-onna so I think the Girl's there are the past version of Kuchisake-onna.
Love how Kuchisake and sei’s relationship is slowly growing and I can’t wait to see them tackle the future together. Praying that they can get married and have a child (if it’s possible for them) by mangas end.
Kuchizake-onna could really use a cute name, rather than her monster-race type, she's a cute girl after all, even when she hinges her jaw, so she deserves a cute name.