Kei Marume, a Kyushu native, transitioned from the Earth Defense Army’s Operations Staff Office to the Science Guard Party after directly petitioning Captain Akiyama for membership—a unique recruitment path that bypassed formal recommendations. His sincerity and fervor often drove him to voice opinions bluntly, sparking occasional team friction, though his authenticity solidified trust among peers.

A sharpshooter renowned for physical prowess and tactical ingenuity, he approached missions with analytical precision, complemented by technical skill in crafting devices like the Dimension Compass to assist a displaced interdimensional being. Despite frontline confidence, his hidden affection for Mutsumi Hoshikawa exposed vulnerabilities, culminating in a misguided pendant gift that imperiled her during a mission—a mistake he urgently remedied through intervention.

Marume’s skepticism fixated on Choichiro Hikari’s erratic disappearances during pivotal events. He publicly challenged Hikari’s dependability, probing his ties to Ultraman Joneus until the Heller Empire crisis forced a confrontation. Here, Marume unmasked Hikari’s secret identity, directly witnessing Joneus’ transformation.

Compassion tempered his intensity, evident when consoling a child grieving a slain monster companion. Yet impulsiveness occasionally overrode discipline, as seen in his rogue engagement against a regenerating kaiju. His defining growth emerged during the Heller invasion: abandoning reliance on allies, he independently strategized U40’s liberation without Ultraman’s aid, cementing his shift from doubting soldier to autonomous defender.