Girl's Backdoor

Girl's Backdoor (少女使い（ガールズバックドア） Shōjo Tsukai (Gāruzu Bakkudoa)) is a device capable of manipulating girls presumably created by Biondetta Shiroyama.

Principles
Girl's Backdoor looks like a simple black glove. The target is set by wearing the glove and pointing at the target’s face or head. When Girl's Backdoor is activated several red threads shoot out of the glove. The ends of the threads contain thick phantom blades resembling bear traps or a human hand and they tear into the victim' arms, legs, and torso one after another. Only the victim and the user can see and hear the threads, and the victim can't speak out or move despite the intense pain they feel.

Girl’s Backdoor works by inputting data. The target is controlled with tree-style settings that provide a programmed flowchart of what to do when certain things do or do not occur. When inputting the data, the user can move the fingers of the glove to display a keyboard interface in empty air, though simple commands can be given by simply ordering the victim, such as telling her to "Get lost" or "Shut up". The flowchart can also be formed as a combination of movements: run, jump, land, etc. When putting together the flowchart, there is no need to start from scratch and mention every little movement of the target’s fingers. The user can draw on and use the target’s skills and knowledge. For example, telling a chef to cook a meal or telling a safe keeper to open the lock.

Girl’s Backdoor can control up to three people at once. If the user attempts to control a fourth or more, their existing stock will be released in the order at which they were initially controlled. Girl’s Backdoor can only control girls and young women between the ages of 10 and 25. There are three ways for someone controlled by Girl’s Backdoor to be released: the user announcing that they are released, the user attempting to control a fourth person, or the controlled person being knocked unconscious. When someone controlled by Girl’s Backdoor is released, they lose their memories of being controlled. The control can be spread to a third party from a woman already being controlled, creating a chain. Releasing a member of a chain will also release any woman controlled by her.

Girl's Backdoor was initially speculated by Maria Heartocean to work similarly to a vessel’s ability to intervene in a Material’s mind and take limited control of the Material. The mental capacity of a Material is so much greater than a human’s that the vessel can never take complete control, but Maria estimated a human could take nearly perfect control when the target is another human. However, as revealed later by the Red Lady, it works by using a single hair of the White Queen. The glove sends the White Queen’s hair into the user's body to connect their soul to hers and borrow a portion of her authority. Simply put, it is used to become one with the Queen to borrow that authority. It obtains the target's obedience through fear.

Users

 * Kuina Murasame
 * Biondetta Shiroyama
 * Youji Makihara and eight others

Light Novel Volume 03
Rumors of Girl's Backdoor, a mysterious tool able to control teenage girls, exploded over the internet, though nobody seems sure about its shape or mechanics. Several fake Girl's Backdoor were sold in online auctions.

Multiple Girl's Backdoor were handed over to random people to cause a diversion in Toy Dream 35 and draw attention away from Biondetta and Kuina, who also made use of Kuina's own Girl's Backdoor. Unbeknownst to Kuina, Biondetta had her own Girl's Backdoor that she used to manipulate her.