Eiichiro Mishima is a minor but pivotal character in the Kyoto Inferno storyline, serving as the older brother of a boy named Eiji Mishima. He was born and raised in Shingetsu Village, a small rural town that falls under the brutal control of the villainous Shishio Makoto and is ruled by his henchman, Senkaku. Eiichiro worked as a covert agent, or undercover spy, for the police force, taking direct orders from the formidable officer Saitō Hajime. His mission was to infiltrate his own occupied hometown to gather and relay intelligence on Shishio's activities to the authorities.

In terms of personality, Eiichiro is defined by his deep devotion to his family and his sense of responsibility. Despite the dangerous circumstances of his undercover work, his primary motivation is not duty or revenge, but the protection of his loved ones. He cares deeply for his parents and his younger brother, Eiji, and desperately wants to shield them from the horrors of Shishio's regime. His actions are driven by a selfless need to save them, even when faced with overwhelming odds.

Eiichiro's role in the story is brief but serves as a key catalyst for events. After learning that the Meiji government plans to abandon Shingetsu Village and leave its people to Shishio's machinations, he takes the desperate risk of trying to smuggle his family to safety. Unfortunately, his escape plot is discovered by Senkaku. Fatally wounded in the ensuing attack, Eiichiro manages to flee into a nearby forest with his brother Eiji before succumbing to his injuries. He is found there by the series' protagonist, Himura Kenshin, and the young ninja Makimachi Misao. With his dying breath, he uses his last ounce of strength to beg them to save his younger brother and his besieged hometown.

His key relationships are centered on his family and his commander. His bond with his younger brother, Eiji, is the most significant, as his death and final plea directly shape Eiji's subsequent desire for revenge and the boy's emotional journey. His professional relationship with Saitō Hajime, for whom he worked as an undercover agent, is also notable. Following Eiichiro's death, Saitō shows a rare moment of solemn respect, indicating the value he placed on his fallen operative.

As a character, Eiichiro does not undergo development, as his death occurs shortly after his introduction. However, his sacrifice serves as a major development point for others. His demise fuels his brother Eiji's initial thirst for vengeance and highlights the brutal reach of Shishio Makoto's power, while also reinforcing Kenshin's commitment to protecting the innocent. Regarding notable abilities, Eiichiro is shown to wield a sword, but his skill level as a swordsman remains unknown as he is quickly and mortally wounded by Senkaku before any real fight can occur.