Yuuki Yuuna wa Yuusha de Aru - Jukai no Kioku

Sanshuu Middle School 2nd year, Yuuki Yuuna.
Cheerful, energetic and always optimistic, she's a normal girl that is a member of the "Hero Club", a club that does anything it can to help people out.
With the 3rd year Inubouzaki Fuu at its center, the Hero Club's activities include all sorts of things that benefit the public, from "finding foster parents for kittens" to "helping out at the kindergarten".
However, the girls of the Hero Club have a true, secret duty:
To fight mankind's natural enemy, the "Vertex", in a different dimension that regular people don't know about, the "Jukai".
Only chosen Heroes have what it takes to fight the enemies that invade the Jukai to bring an end to the human world.
...Or that's how it's supposed to be, but the Jukai seems to be a bit off lately...?
Who is the mysterious figure they encounter within this space where only Heroes and Vertex should be able to move...?