The real Mashiro Blan de Windbloom is shown dead on the first page of chapter 1. The Mashiro for the rest of the story is a boy, real name unknown, who impersonates the real Mashiro under Sergay August Taiki's orders. Mashiro first appears on the second page of chapter 1, recently arrived in Wind City. He disguises himself as the princess and meets his assigned escorts Arika Yumemiya and Nina Wáng. They encounter a member of Schwarz who attempts to assassinate Mashiro, causing Mashiro to make a contract with Arika with the Sky Blue Sapphire.
Mashiro makes it to Garderobe safely; in chapters 2 and 3 he meets with the Garderobe staff, who already know of his true identity, and the Garderobe students. The next few chapters deal with his school life, his growth as a leader, and his relationship with Arika, Nina, and Erstin Ho. Nina discovers his real gender in chapter 10, causing her to think differently of him and his motives. Eventually, Mashiro's concern for Arika, who leaves in chapter 20 to find her parents, leads Mashiro (along with Nina and Erstin) to follow her to the Black Valley.
At the Black Valley, Mashiro shows his resolve as a leader while confronting Cardair and protecting the refugees. He returns to Garderobe in chapter 24 after Garderobe staff stop an invalid butou between Arika and Cardair's Otome.
After Erstin finds out about his gender in chapter 25, Mashiro is summoned to Garderobe castle and informed by Sergay that he will be crowned queen. Schwarz attack at the coronation, causing Sergay to send Mashiro and Arika through a secret passage. The two arrive at an underground chamber, where the real Mashiro awaits.
In chapter 30, Sergay reveals that the real Mashiro's death was a suicide intended to grant her stronger magical powers when she was resurrected. In the following battle, Nagi de Artai is fatally hit and gives his ring to Mashiro, making him the master of Nina.
The real Mashiro gets Fumi to revive the HiME Mai Tokiha in chapter 32. The subsequent summoning of Kagutsuchi causes flashbacks for the fake Mashiro. He and his retinue are rescued by Schwarz and Erstin and taken to the Windbloom slums to regroup. In the following chapters, Mashiro heads to Garderobe while Nina, Erstin, the city police, and the Garderobe Otome hold off enemy forces.
Mashiro is tormented by the fact that the others are sacrificing their lives for him, but is comforted by Nina before she becomes the last one to leave him in chapter 36. At this point Arika returns to protect him from Mai, but Arika's power runs out quickly.
Mai attacks Mashiro, but Rena arrives and is hit instead. Rena tells Mashiro how to unlock the true power of the Blue Sky Sapphire, which Mashiro and Arika use to send Mai and Mikoto retreating, and the location where the real Mashiro and Sergay are waiting - the Crystal Shrine. The Garderobe Otome hold off the other resurrected HiMEs to allow Mashiro, Arika, and Nina to reach the Crystal Shrine.
The real Mashiro uses her powers to trap the fake Mashiro in an illusion of his old life. However, Mashiro is able to escape in chapter 42 with the help of Yuuichi Tate's spirit, gaining the Pure White Diamond, which he uses to defeat Mai. Sergay reveals the truth that the two Mashiros are twins and both heirs to the throne. The real Mashiro is then killed by Fumi.
Arika, Nina, Erstin, and Mashiro are able to defeat Fumi and Sergay, but Sergay manages to wound Mashiro and steal the Pure White Diamond. In the final chapter, the purified spirits of the real Mashiro and Fumi appear to Mashiro and send him back to the land of the living. Gathering the power of the Otome, Mashiro materialises a Robe and defeats the monstrous Sergay.