Sakura captures the Mirror card in the third volume of the manga. A mirror of Sakura appears and starts causing trouble around Tomoeda! Sakura's friends are worried about her, and can hardly believe that she would do such bad things!
Sakura and Kero-chan have a hard time trying to figure out what might be going on. Kero-chan teaches Sakura how to use the cards she has captured to do a reading. The result doesn't make much sense to Sakura at first. As she is thinking about it, she turns over the Flower card. She sees peach blossoms in her mind, and gets a bad feeling. She senses that her brother is in danger!
Touya had run into the false Sakura in town, and she told him that she lost something, and asked him to help her find it. Touya goes with her to look for her missing thing. The false Sakura leads him to a wooded area at the edge of a cliff. Sakura and Syaoran rush to the cliff, but when they get there, Touya has already fallen off! Touya knew that the Sakura he followed wasn't his sister; he could sense that she wasn't human. He went with her because he wanted to help her find peace. Mirror was stunned by his compassion and when Sakura arrived, Mirror helped her to figure out which card she was by copying all Sakura's movements. Sakura was able to catch the card once she could name it. After a tender scene where she kisses Touya good-bye with tears in her eyes, Mirror returned to her card form.
Basically the same thing happened in the anime, but, of course, it wasn't nearly that cool. ^_^
Mirror is actually a very nice card. She wanted to cause trouble for Sakura at first, but after meeting Touya, Mirror becomes much more compassionate and kind.