Aria the Scarlet Ammo

Kinji Tohyama is a boy plagued by Hysteria Savant Syndrome, a condition that runs in his family which enhances their members when they're aroused. He has always wanted to give up being a Butei (Armed Detective) for a normal teenage life. All chance of this goes out the window when he gets caught up in a bike bombing by the Butei Killer. He is saved by a girl who fell from the sky, later transferring to his class and introducing herself as Aria Holmes Kanzaki. Apparently, she intends for Kinji to become her "slave".