Bronk Stone, known as Tetsuo Takeda in the original Japanese version, is a main supporting character and a student at Heartland Academy, where he is a classmate and close friend of the protagonist Yuma Tsukumo. At the age of thirteen, Bronk has a unique and somewhat stout appearance, distinct from many of his peers, and is most often seen wearing a backwards cap with his hair sticking out from underneath. He lives with his mother, Tetsumi Stone, and his older sister, Brianna Stone.

Bronk's personality is a blend of competitive pride and genuine loyalty. He is an adamant and sometimes arrogant individual who takes a certain pleasure in the embarrassment of others, often goading Yuma into challenges he cannot win and laughing at the resulting failures alongside their classmates. This initial behavior stems from his early days as a bully, but he quickly evolves into one of Yuma's most steadfast allies. Despite his gruff exterior, Bronk is strong-willed and has difficulty openly expressing his emotions. For example, after retrieving the broken half of Yuma's pendant, he downplayed the act by claiming he simply stumbled upon it, rather than admitting he had searched for it to help his depressed friend. His motivations are largely rooted in friendship and a sense of personal honor; he deeply values the bonds he forms and despises dishonesty, which is demonstrated by his anger when Yuma breaks a promise not to use the dangerous Number cards.

Within the story, Bronk serves as a vital member of Yuma's support network. He is a founding member of the Numbers Club, a group formed by Yuma's friends to assist in gathering information on the mysterious Number cards without directly involving Yuma in their investigations. Throughout the series, he provides consistent emotional and tactical support from the sidelines. One of his most significant contributions occurs when Yuma is possessed by the malevolent Number 96: Dark Mist. Bronk courageously duels his possessed friend, using Yuma's own Number 39: Utopia to break the spirit's control. During this ordeal, Bronk demonstrates an incredibly strong will, as he is able to shake off the corrupting influence that typically affects those who wield a Number card, a feat possibly aided by his temporary possession of the Emperor's Key. His key relationships include his friendly rivalry and unshakable loyalty to Yuma, his close friendship with Tori Meadows, and a notable, persistent crush on Rio Kastle, which becomes a more prominent part of his character development in the later series.

Bronk is a capable Duelist whose skills are often highlighted by his long-standing winning record against Yuma. He specializes in a Machine-type deck, primarily focusing on the Gearspring or Clockwork archetype. His strategy revolves around the manipulation of special counters to increase the attack power of his monsters and uses cards like Iron Call and Iron Draw to maintain advantage on the field. His signature ace monster is Tin Archduke, a powerful Machine-type Xyz Monster that represents his resilient and mechanical strategy. As the story progresses into the Barian invasion arc, Bronk faces more dire consequences for his loyalty. He willingly follows his friends into the Barian world to fight alongside them. In a particularly tragic turn, he duels against Marin, the Barian form of Rio Kastle, for whom he has deep feelings. He is defeated and has his memories sealed away, only to be revived and restored following the final defeat of the Barian Emperor Nash. His journey concludes as he returns to his normal life at Heartland Academy, having grown from a boastful rival into a mature and reliable friend who stood at the forefront of a multidimensional conflict.