Age: 11 (in appearance)
DOB: January 20
Height: 138 cm
Weight: 31 kg (68.2 lbs)
Blood type: AB-
Mashiro first appears unnamed during episode 2 in the flower garden with Fumi when Mai Tokiha passes by and asks them as to where Mikoto Minagi scampered off to. Mashiro and Mai formally meet in episode 3, when Mashiro tells Mai that she is gathering the HiMEs to defeat Orphans. Mashiro tells her to decide what she wants to do on her own, which later becomes a recurring theme in Mai's story.
Beginning in episode 8, it becomes apparent that Mashiro is hiding facts from the HiMEs, and that she feels guilty for putting the HiMEs in the position they are in. In episode 12, she pointedly refuses to tell Mai who is behind the vampire attacks at the Academy, although she later calls Yukino Kikukawa and asks her to help find the vampire.
In episode 14, Mashiro confronts Alyssa Searrs, who calls her the "Queen of Hell." After their talk Mashiro's body opens to reveal an Orphan, which is defeated by Miyu Greer and the body is shown to be a doll used as a decoy.
Mashiro meets with the heads of the First District in episode 17, then returns to Fuka Academy in the following episode and is confronted by Midori Sugiura, with the information that the real Mashiro Kazahana died in an excavation accident. Midori threatens Mashiro but she is blocked by Fumi, who is revealed to be a HiME herself.
In episode 21, Mashiro gives Midori the key to the room where Miyu is recovering. Mashiro and her maid then go to face the Obsidian Lord, Reito Kanzaki, who calls her the Crystal Princess. It is revealed that Mashiro was somehow involved in the previous HiME carnival and that she once had feelings for the Obsidian Lord. She called him 'Onii-sama', "Older Brother" in Japanese, suggesting that she was close to the Obsidion Lord, or related to him. Before she tansforms into her Child form, Reito asked Mikoto, 'You would do anything for me, won't you, my little sister?', prompting a shocked reaction from Mashiro, who seems to suggest that he has been the brother of a HiME before (although not necessarily hers) by controlling time and/or reality. This way, he would not have to die at the hands of a HiME. However, as the winner of the HiME Carnival is entitled to marry the Obsidian Lord, it makes more sense to assume that Mashiro had feelings for him in the past, and merely referred to him as a brother affectionately, as Shiho does with Yuuichi. Mashiro then transforms into Fumi's Child, Suishou. Their attack ends in defeat and Mashiro's doll body is hung above the Obsidian Lord's throne.
Mashiro's true body is revealed in episode 25, encased in a crystal star. She is also further revealed to be the winner of the previous HiME carnival as well as the former user of Kagutsuchi, Mai's Child. In the final episode, Miyu's anti-materializer frees Mashiro, and she floats up to the Crystal Shrine. She revives the fallen HiME, resurrecting them and their loved ones, and restoring their HiME powers. In the ending credits, Nagi and Mashiro (still in a wheelchair) enter the large doors at the far end of the library.
In episode 21, Mashiro displays two abilities: the ability to create a barrier preventing eavesdropping, and the ability to transform into a Child. As Fumi's Child, Suishou, she radically changes appearance and wields a double-sided blade, and bears Kagutsuchi's crest of three tomoe on her robe. In her true body in episode 26, she is able to appear in spirit form to multiple people, revive the dead, and restore the powers of fallen HiME.