Masaru Okui is a supporting character in the anime series Argevollen. He serves as a Lance Corporal and later a Senior Private in the Arandas Army's 8th Independent Unit, functioning as an infantry soldier who provides ground support for his squad. His role is that of a backup soldier rather than a primary Trail Krieger pilot.

Okui is characterized by a hotheaded and stubborn temperament. He is quick to voice complaints and often acts on his strong sense of justice, which leads him into conflicts with others. His impulsive nature and tendency to speak his mind sometimes cause problems within the unit. He openly criticized the protagonist Tokimune Susumu's reckless sense of justice, believing it placed the entire unit at unnecessary risk.

His motivations stem from a desire to see the unit operate effectively and safely, though his confrontational approach can create friction with his comrades. Okui's personal background is not extensively detailed in the series; he is depicted as a career soldier dedicated to his duties.

Key relationships within the story include his bond with the other members of the 8th Independent Unit, particularly his interactions with Tokimune, whose idealism he initially opposes. Over the course of the series, Okui's development is subtle: he remains a supporting military figure who continues to fulfill his role in ground combat, and his initial antagonism toward Tokimune softens as they fight together.

Notable abilities include his competence as an infantry soldier, contributing to the unit's effectiveness in ground engagements. He does not pilot a mecha but rather operates as a conventional soldier, providing support with small arms and tactical awareness during missions. His physical appearance includes brown hair and brown eyes. No major character arc or transformation is evident from the available information; he remains a steadfast, if argumentative, member of the team throughout the series.