Masaki Amakasu is a well-built youth in his late teens or early twenties. He sports long dark blue hair tied back, marked by a distinctive light blue lock at the front and ends. His black armor identifies him with the Uesugi Bushi elite squad, accented by a yellow cloth piece at his waist.

Originally from a small mining town under Oni sympathizers' control, his life shifted at age nine when Uesugi Bushi liberated his home. He then joined the Band of Bushi, remaining with them since. His childhood name was Misaomaru, though its significance remains unexplored.

Within the Uesugi Bushi, Masaki regularly assists Naoe Kanetatsu, notably during the Awaji Island Recapture Operation. He manages logistical duties like combat records and tax documentation review. However, his occasional imprecision in reporting minor Oni kills—such as citing "three or four" without exact tallies—draws reprimands from the meticulous Kanetatsu.

Masaki engages directly in combat against Oni threats. During the Awaji campaign, he battles lesser Oni alongside Kanetatsu and fellow Bushi, aiding defenses against larger Oni forces assaulting allies like Musashi and Michiru. He later helps protect a local castle during a heightened Oni attack, where Uesugi forces prevail after reinforcements led by commander Tatsuomi Uesugi arrive. In a tactical moment, he intervenes in Kanetatsu's clash with Inukawa, creating an opening for Kanetatsu to counter and defeat the opponent.

His combat skills include wielding a standard Kitetsu sword, though specific techniques or unique abilities remain unelaborated.