Aoi Tachibana, 15 years old, acts as the "straight man" within her group, particularly countering the eccentric behaviors of Jun and Zill. She maintains a grounded, rational demeanor, offering logical counterpoints to their antics.

A key aspect of her identity is a concealed passion: Aoi is a fujoshi, privately enjoying boys-love manga. This interest remains a secret from others. Her personal space serves as a private sanctuary, with shelves dedicated to her collection of BL manga. This duality – her outward role as the rational foil versus her hidden fandom – defines her.

Her story unfolds against humanity's battle with the Firow, a mysterious alien race that appeared on Earth in 20XX. "X Maidens," individuals wielding specialized weapons, combat this threat. While Aoi is identified among the main characters in this conflict, specific details about her combat involvement, weapon usage, or operational role within the X Maiden framework remain undefined.