Kouta Tatsumi is the younger brother of Kimari Tatsumi and the son of Denjirou and Hanayo Tatsumi. Though not a primary card battler, he actively collects Battle Spirits cards with a dedicated focus on acquiring Pentan-themed spirits. He frequently trades cards to expand his Pentan collection and passionately desires to become a Pentan master, even wishing for a Pentan costume.

He admires Hajime Hinobori and often accompanies him during events, though his non-battler status limits his direct role in major plot points. Kouta experiences consistent losses against his sister Kimari, contributing to his reluctance to take a more active competitive role. Despite this, he participates in several minor or background battles: he lost to Arata Yakushiji in episode 23, had an undecided battle with Hajime in episode 28, and won against Kobushi Nigiri in a background fight during episode 49.

Kouta exhibits a playful streak, occasionally teasing his sister Kimari. His intense enthusiasm for Pentan spirits parallels Kotarou from *Battle Spirits Shounen Gekiha Dan* and their similar obsession. His narrative role remains supportive, with activities centered on card collection and minor comedic interactions rather than tournament progression or personal growth arcs.