But despite that, her Yandere self still feels like only another layer below her bubbly perfect-gf self. From how Old F Tunnel turned out, she's not as unknowable and inhuman as she portrays her "true self" to be. Given her genuine approach to trauma-levels of fear, and it leading to the quick search for confirmation of her true feelings on the supernatural, I'd say she's still pure at heart.
Look past the Yandere and Eiko is a lovely girl who cherishes Keitarou, Yayoi-chan, and scary stories. She's just effective at making it look super scary.