Masaki Amakasu hails from a small mining town once controlled by Oni sympathizers, liberated by the Uesugi Bushi a decade before the main events when he was nine. He subsequently joined their ranks, ascending to the elite squad serving directly under Kanetatsu Naoe, a first-class general overseeing territorial revenue.

A well-built young man in his late teens or early twenties, Masaki possesses long, dark blue hair typically tied up, featuring a distinctive light blue streak at the front and ends. He wears the standard black armor of the Uesugi Bushi elite, accented by a yellow cloth tied around his waist.

Masaki exhibits a strong sense of duty, discipline, and adherence to rules. Reserved and introspective, he focuses on concrete details and facts during missions. This meticulous nature is evident in his administrative support for Kanetatsu, such as maintaining precise battle records during the Awaji Island Recapture Operation. He expressed visible frustration over vague combat reports, emphasizing the critical need for accurate documentation over estimations, valuing established procedures and organization.

During the Awaji Island operation, Masaki fought against Oni forces, witnessing Kanetatsu's tactical leadership firsthand, including efforts to maintain troop formations against multiple Oni. He later assisted in defending Musashi, Michiru, and Akihiro from Oni attacks alongside other Uesugi Bushi, contributing to broader mission objectives under Tatsuomi Uesugi's command.

His background includes a childhood name, Misaomaru, though early family details beyond the liberation remain unspecified. The surname Amakasu potentially signifies a fictionalized lineage connected to historical figures like Amakasu Kagemochi or Kagetsugu, aligning with the series' theme of legacy and descendants.