Galatea (ガラテイア Garateia), also known as the Doppelganger (ドッペルゲンガー Dopperuganga), is an antagonist of Hollow Mirror Field story line. She kidnaps people from Ayamegaoka to devour their life energy. It is later revealed that Kanae Kuroshiba made her do this in order to do the dirty work of kidnapping people and sacrificing them to complete the Artificial Emerald Tablet.
After having arrived in Japan, Kanae Kuroshida created the Artificial Emerald Tablet and planed for world domination once it was completed. Due to the fact that the Artificial Emerald Tablet needed the life forces of ten thousand humans, she created the Galatea to do the job of kidnapping and specifically modified spherical stone that allow the toucher to bypass the effect of her Specialized Bounded Field and put it into Galatea's core, so that it can cast the spell to transfer victims' life force into the Artificial Emerald Tablet floating on the sky of her Specialized Bounded Field.
In the year of 2009, the Artificial Emerald Tablet was almost completed. Galatea was still on her job of kidnapping people from Ayamegaoka. Unfortunately, she targeted Kaori Natsuki and the latter was worried of seeing her own doppelganger. Kaori thus told Tadashi Teruya and who in turn asked Shuu Amami to investigate the matter. Galatea shapeshifted again into Kaori in October 1 at Ayanas and it caught Shuu's attention. The next day, she infiltrated Kouryoukan Academy to kidnap Kaori, but was almost caught by Shuu and was forced to leave the Academy.
Knowing that she could not target Kaori any longer, she headed for Ayanas and chose Shione Azuma, who was there alone, as her new target. Shuu and the gang later came to Ayanas, but was bewildered as there were two Shione standing before them. With a question, Shuu was able to identify the fake Shione and prepared to strike. Galatea activated the Specialized Bounded Field, which disabled the use of magic inside its perimeter, and kidnapped Shione, leaving the Shuu, Mio was shocked as they were unable to activate their spell while Shiori laid frozen on the ground.
Galatea then brought Shione to the Kouryoukan Academy's schoolyard, chained her to the ground and performed a spell to draining Shione's life energy. A short while later, Shuu arrived and, to her surprise, battled her with his bare hands using a martial art technique invented by his grandfather and were being strengthened by the life energy borrowed from Mio Kouno. When they were battling, Tadashi came riding a CBR400 crashing into her. The crash did not affect her much, but it distracted her from Shione, giving Kaori a chance to unchain her. Realizing her prey was getting away, Galatea turned back to attack her, but Shuu continued hitting her, leaving her no time to catch Shione.
Desperate, Galatea opened the lock of her core, revealing her true body－a small homunculus－and attacked Shuu with white energy beams fired from her mouth. Shuu managed to calculate the cooldown time of Galatea's energy shot and managed to land a hit on her. Upon hitting her homunculus body, Shuu felt the surge of magical power flowing into his body and began to activate his spell, which defeated her in one hit.
Shuu then used a spell to hack into her mind to find out more information. He was unsuccessful in finding out the true master of the Specialized Bounded Field but it was revealed that she was manipulated by her master into doing her job by a promise of turning her into a real human after the job is done. In addition, it was revealed that the previous apostle of Index who had been monitoring Ayamegaoka, Agatha of the Silver Cross, who was also a candidate for the next 14 saints, was one of the victim of Galatea. It was Agatha sudden disappearance that Index sent Shiori, or rather, Ursula of the Bookshelf, to fill the job of monitoring the place and at the same time, to investigate her disappearance.
Galatea's most powerful aspect is her shape-shifting ability, which enables her to make her victim confused and panicked and easier to kidnap. Aside from this, however, she was only slightly equipped with combat abilities and, therefore, wasn't much of a fighter against magi. To compensate for this drawback, her core, which was spherical stone modified by Kanae, enables her to call forth Kanae's Specialized Bounded Field, which can disable the use of magic in its area of effect but still allow her to cast spells. In case of an emergency, Galatea can reveal her true body and attack by firing white energy balls from her mouth.