Tasuku Ryūenji begins as a seventh-grade member of the Buddy Police before taking leave to focus on studies. His past is marked by "The Disaster," a tragedy claiming his family, where he was rescued from mountains alongside his future buddy, Jackknife Dragon. This loss fostered his belief that adults overly shelter children, driving his desire to prove independence and maturity early, leading him to join the Buddy Police in pursuit of justice. During Season 1, Tasuku wields a Dragon World deck featuring Jackknife Dragon as his buddy monster and utilizes the core gadget "Star Pulser." His missions include apprehending criminals and delivering core deck cases, notably to protagonist Gao Mikado. After losing an unofficial match to Gao, Tasuku remains positive, declares him a rival, and gifts him the card "Gargantua Punisher" as a sign of respect. He demonstrates strategic skill in operations, confronting antagonist Wolf at the Photon Metal Mine and employing restricted "Future Force" abilities against regulations to neutralize threats. Corrupted by Kyoya Gaen, Tasuku's sense of justice falters, and he joins the antagonist group Disaster under the alias "Purgatory Knight." In this phase, he adopts a Darkness Dragon World deck with Demios Sword Dragon as his buddy and utilizes dark armor as his core gadget. Motivated by disillusionment with the Buddy Police's perceived inaction and a conviction that adults perpetuate societal problems, he embraces a philosophy that children must seize power. This leads to extreme actions, including attempting to sacrifice Jackknife to achieve his goals. His identity is revealed during a confrontation with Noboru Kodo. Following his departure from Disaster, Tasuku experiences intense guilt and shame, feeling unworthy of being Jackknife's buddy. Jackknife's encouragement aids his recovery, and he temporarily steps back from the Buddy Police. He trains in Dragon World during this period, evolving his deck to Star Dragon World. Jackknife undergoes repairs and upgrades, becoming Star Guardian Jackknife and later Star Dragoner Jackknife. In Season 2 (Future Card Buddyfight Hundred), he assists in combating the Hundred Demons invasion while refining his new deck. In Season 3 (Triple D), Tasuku unlocks the "Dragon Force of Joy" ability, marked by crystalline aesthetics and luminous energy. This evolves into "Style of Justice," enhancing his Star Dragon World capabilities. He partners with Gaito Kisaragi on missions, forming a complementary duo where Tasuku provides strategic acumen. An emotional reunion with Jackknife, facilitated by Gao Mikado and Bal Dragon, triggers his Dragon Force activation through tears of joy. By Future Card Buddyfight Ace, Tasuku has risen to the rank of Commissioner within the Buddy Police, overseeing high-level operations. His relationship with Jackknife remains foundational, described as familial due to their shared history and mutual sacrifices. Personality-wise, Tasuku exhibits a cool, mature demeanor and is widely admired for his kindness. He strives to appear adult-like, avoiding typical childhood activities to focus on responsibilities. However, he displays occasional airheadedness, such as misnaming Noboru’s "Kitten Shirt" or overlooking basic buddyfight protocols. His resolve to protect others sometimes leads to defying orders or reckless self-sacrifice. Deep-seated trauma from "The Disaster" contributes to stubbornness and an inability to accept personal failure, particularly evident during his Purgatory Knight phase. Despite conflicts, he forgives easily and values redemption for others.