Gai Usui is a minor character from the Cardfight!! Vanguard anime, recognized as a member of Team Caesar and the younger brother of Yuri Usui. He is physically distinguished by his tall stature, being the tallest member of his team, and his tan skin. Gai has long navy hair that he ties into a low ponytail with a sky blue ribbon, and his eyes share the same color as this ribbon. His typical attire consists of a yellow full-sleeve shirt with the sleeves folded to the elbow, a pair of black jeans, and a green sleeveless jumper that features frills on the sleeves and hem. He completes his look with dark brown boots that rise just above the ankle, a light brown belt, and a necklace featuring a silver rectangle, similar to a dog tag.

Within Team Caesar, Gai adopts the persona of the Gladiator and styles himself as the personal bodyguard to his teammate, Koutei. This self-appointed role means he often takes the field as the team's vanguard fighter before his other teammates. His personality in fights is direct, as his favored strategy involves reinforcing his Vanguard with supporting units to build a strong formation before launching a decisive strike, a tactic he shares with Koutei. However, his aggressive approach is not without flaw. His sister, Yuri, frequently chides him for lacking a clear winning image when he enters a cardfight, a criticism that is reflected in his battle record where he has suffered several key losses. For instance, he has lost to fighters like Kamui Katsuragi and the formidable Toshiki Kai, suggesting that his tactical approach, while powerful, can be vulnerable against adaptable or highly skilled opponents.

His role in the story is primarily as a supporting character, often appearing alongside Team Caesar to provide opposition for the main characters. His development is limited, but his relationship with his sister Yuri is a defining aspect of his character, as her critiques highlight a significant gap between his confidence as a bodyguard and his execution as a fighter. A capable fighter, Gai favors a deck from the Dimension Police clan, which focuses on powerful robot units. His ace card is Super Dimensional Robo, Daiyusha, a unit that embodies the kind of reinforced, powerful Vanguard he strives to build. His record demonstrates he is a competent fighter capable of winning matches, such as against Katsumi Morikawa, but his lack of a consistent winning strategy, as pointed out by Yuri, prevents him from achieving greater success against the series' more prominent fighters.