Gouki Daimonji is a third-year student at Tokyo Metropolitan Harumi High School and serves as the leader of the team he commands, which is known as Team Handsome. Embracing a self-styled captain pirate persona, his appearance reflects this theme. He wears a red bandana over his medium-length black hair and has distinctive orange eyes. His attire typically consists of a brown vest over an orange and white torn shirt, a red sash belt, baggy tan pants, and knee-length brown boots. Beyond his role as a student and team leader, Gouki also presides over Card Shop Voyage, a position through which he welcomes and earns the respect of many aspiring cardfighters.

In terms of personality, Gouki is excitable and straightforward, possessing a notable lack of intellectual sharpness. Despite this, he is a well-respected figure in the Vanguard community who is always eager to accept a cardfight, regardless of an opponent's skill level. He also demonstrates a capacity for reciprocal respect, such as when he welcomed another fighter to his shop with open arms. One of the most defining aspects of his personality is his relationship with his younger sister, Nagisa. Gouki dotes on her excessively, which has contributed to her spoiled nature. He displays a marked submissiveness in their interactions, a trait that often leads to comedic situations. His devotion to her happiness is a primary motivation for his early actions, as a misunderstanding leads him to falsely believe that another character, Kamui Katsuragi, is avoiding Nagisa's proposals due to a relationship with Aichi Sendou. Acting on this belief, Gouki confronts and accuses Aichi of being a home wrecker, challenging him to a fight in an attempt to win Kamui back for his sister.

Within the story, Gouki's primary role is as a competitive rival to the main characters and a recurring tournament opponent. His team competes in regional and national tournaments, which includes facing the protagonists' team in the regionals where Gouki suffers a loss to Aichi, resulting in his team's elimination. In a later regional tournament, his team is defeated in the semifinals by another team, preventing a rematch. Prior to the events of the anime's main storyline, Gouki secured second place in a major championship, establishing his high-level skill early on. During another story arc, his team vies for the national championship with Kamui temporarily joining their ranks. Witnessing the strategic use of a specific limit break ability inspires Gouki to adapt similar tactics into his own fighting style. Throughout the series, Gouki continues to be an active tournament participant and remains a familiar presence working at his card shop.

Gouki is a dedicated master of the Granblue clan, a deck built around resurrection mechanics that allow him to continuously revive units from the drop zone. This strategy provides the distinct advantage of allowing aggressive guarding without the permanent loss of the guarding unit. His playstyle is notable for a signature move where he superior calls his first vanguard to the front line for an early, free attack, only to later replace it with more powerful resurrected units. He often favors the left side of the field for these placements and uses superior ride lines to conserve card advantage while pressuring his opponent with power-gaining units. His ultimate maneuver involves filling his soul with specific units to activate the megablast of his ace unit, King of Demonic Seas, Basskirk, which allows him to superior call up to five units from the drop zone for a devastating multi-attack.

Over the course of the series, Gouki's development is reflected in the evolution of his deck. His initial deck, which he calls Infinite Resurrection Hell, was centered on Basskirk and included support units like Captain Nightmist and Ruin Shade. After being inspired by another fighter's limit break, Gouki developed a new deck, which he calls Criminal Strom. This updated strategy focuses on a different unit, Ice Prison Necromancer, Cocytus, and uses a new first vanguard to accumulate resources in the drop zone while maintaining card advantage. This adaptation showcases his ability to learn from other skilled fighters and refine his strategies in response to new challenges.