Valkyrie Drive: Mermaid

Synopsis

Stressed from work, Akira, who is revealed to be a woman, sneaks out to spend some time with Torino, keeping her identity a secret from Mamori and Mirei. Spending the night in Mamori's room, she recalls how her mother arranged for her to act like a man upon arriving at Mermaid in order to stay safe. The next day, some rebellious girls take one of the Ritters hostage in the building where Akira is keeping her male clothes. As Charlotte and Kasumi argue over how to approach the situation, Mamori, Mirei, and Akira follow a secret passage into the building in order to rescue the hostage, only to fall into a trap laid out by the Ritter, who was sick of her current life. With Mirei knocked out, Mamori has Akira drive with her in order to stop the terrorists, briefly exposing her identity to Mirei before leaving the scene.

(Source: Wikipedia)