Until I, a supporting character, become the heroine

High schooler Rui wakes up in a mysterious dungeon, where she meets Claude who saves her from a monster. She realizes that she has been transported to another world, the world of the popular otome game "Tower of God," along with another transmigrator, Mayu, who has a mark proving that she is the heroine. However, Rui has no mark and cannot use magic. In order to return to her original world, mob Rui follows Mayu despite being oppressed by Mayu. Meanwhile, as Rui begins to communicate with the capture targets, her heroine mark begins to appear...?! Taking the place of the heroine in the labyrinth dungeon?! A great reversal-of-love fantasy begins!