Kei Marume, recognized as Lieutenant Marconi in English adaptations, embodies sincerity and fiery passion, often clashing with teammates due to his forthright communication. Yet his authenticity fosters mutual respect, cementing his role as a driven and valued team member. Hailing from Kyushu, he joined his current unit after personally petitioning its leader, becoming the sole member admitted without formal recommendations—a testament to his determination. A former Earth Defense Army Operations Staff Office officer, he boasts technical ingenuity, designing tools like the "Dimension Compass" to aid interdimensional ally Zaanmoth and devising specialized combat strategies.

Pride in his physical prowess and sharpshooting contrasts with quiet vulnerability, particularly around Mutsumi Hoshikawa, whose admiration he quietly seeks through gestures like handcrafting her birthday pendant. His protective instincts emerge during missions, exemplified when guiding young Takashi—unwitting caretaker of kaiju offspring Pero—through danger, balancing tactical resolve with compassion.

Observant and analytical, he uncovers a teammate’s secret identity as Ultraman Joneus, noting behavioral gaps during crises. These suspicions climax during a Heller Empire assault, where he confronts the ally mid-transformation, confirming his theory. Later arcs challenge his reliance on leadership and Ultraman’s power, spurring independent action. During the Heller Empire’s invasion, a daring mission to free an alien planet sees him captured and nearly executed, a crucible that reforges his commitment to teamwork and self-sufficiency.

His persona oscillates between lighthearted humor—providing levity—and tactical brilliance, though occasional recklessness, like unsanctioned kaiju engagements, underscores his unwavering drive to protect. This duality paints him as both a frontline warrior and a layered figure, evolving from impulsiveness to collaborative maturity while retaining his core tenacity.