Jack, a dragon monster known as Jackknife Dragon, serves as the primary buddy monster to Tasuku Ryuenji. His existence is intrinsically linked to the Dragon World, where dragons and humans form partnerships through interdimensional card battles. His narrative significance emerges during a temporal crisis when antagonist Variable Cord forcibly takes Jack through time. This abduction triggers Tasuku's journey into a dystopian future facing annihilation, positioning Jack's rescue as a key motivator for Tasuku. Their bond transcends typical partnerships, exemplified as Tasuku risks temporal displacement to retrieve him, underscoring profound mutual loyalty. In combat, Jack functions as an extension of Tasuku's strategy. His capabilities manifest in associated card mechanics: the "Star Jack Revival" spell grants protective effects like temporary indestructibility to monsters bearing "Jackknife" in their name, demonstrating his resilience. The unit card "Future Card Buddyfight Tasuku & Jack" integrates their partnership, granting synergistic battlefield abilities when deployed together. This card is explicitly designated as "Dragritter, Lateefa," indicating a potential evolved form tied to their combined strength. Jack's character development peaks during critical confrontations, notably in the episode "Shine Bright! Shining Dragoner, Jackknife!!" centered on his transformation. This evolution signifies a power escalation aligned with light and heroism, reinforcing his role as a cornerstone of Tasuku's effectiveness. His presence consistently aligns with themes of interdimensional unity and defending multiple worlds against existential threats.