Deckie emerges from the creature world as an animated deck box, displaced into the human realm during instability within his home dimension. He forms a pact with Joe Kirifuda, serving as the boy's companion and primary conduit for interacting with creature spirits. His physical form is a white and black deck box featuring a face and a red bowtie, capable of manifesting limbs when required. After Joe names a newly encountered civilization "Jokers," a permanent green 'J' mark appears on Deckie's body.

Personality-wise, Deckie bubbles with energy and deep knowledge of the Duel Masters game, frequently guiding Joe. He maintains secrecy about his true nature, pretending to be an ordinary deck box as needed. However, he veers into morally gray territory, occasionally endangering Joe by coercing him into perilous "True Duels" where physical harm is real. This stems from his conviction that true Duel Masters must face extreme trials and his inherent creature-world ethics, which initially prioritize retrieving rogue spirits above human safety. Over time, Joe's influence moderates these tendencies, leading Deckie to reject unnecessary violence against spirits.

Deckie's core abilities focus on card creation and interdimensional travel. He transforms Joe's drawings into functional cards, though his powers have limitations: an initial inability to create dragon-based cards due to their extinction in the creature world, and frequent malfunctions or payment demands from his "Joker" cards. He acts as a conduit to the creature world, swallowing cards to return spirits or physically transporting individuals who enter his mouth. His connection enables him to analyze spirits' origins (natural vs. artificial) and revisit past duelists' memories by processing photographs, albeit within a time limit.

Throughout his journey, Deckie's role transforms. Initially concealing the severity of the creature world's crisis, he later spearheads Joe's expeditions there, starting in the Fire Civilization lands. By resolving conflicts across civilizations—such as aiding the Nature master Kabu Toki and the Water master Cap—Deckie unlocks new abilities: creating Nature Jokers, Water Jokers, and Gacharange Creatures. These expansions allow him to upgrade existing creatures like Jolly the Johnny and Joragon. He also demonstrates deeper historical knowledge, recognizing legacy duelists and creatures from the Kirifuda family archives.

In later conflicts threatening the creature world's stability, including threats like Zero and Gap, Deckie actively collaborates with Joe and other masters to restore balance. His development culminates in overcoming his dragon limitation, producing one from Joe's drawing in a weakened "drawn" state following a decisive battle, signifying his growth and deepening bond with Joe.