The third-ranked warrior was introduced in the game because she plays an important role in the story.
She helps Rhea in overcoming the danger of Valerie.
Canonically, Valerie kills the First-ranked during their confrontation and that battle announces to the world that a massive threat has arisen in the face of a being who once was a human.
Rhea understands the severity of the matter and also acknowledges her lack of strength to deal with Valerie.
The pinkhead realizes, after witnessing the between the new Parasite Chief and William, that defeating Valerie head-on is impossible. She was just beyond her league.
Due to the lack of manpower around that time, Rhea asks for Thea's help in luring Valerie into a trap.
Rhea's specialty is Illusion. She can create the perfect illusions which even fooled a Demon General.
She lured Valerie using the illusion of Austin...and then something happened that Austin didn't want to recall.
