Billy Cassidy is a character who appears as a member of the American team during the Good-Will Games arc. He grew up on a ranch in an unspecified city in the American West, an upbringing that heavily defines his character and playing style. His family relocated to Los Angeles when he was twelve years old, and he struggled to adapt to the fast-paced urban environment.

His primary motivation for playing tennis is not athletic ambition but personal affection. He fell in love at first sight with a girl named Stephanie, who is the daughter of a potential sponsor for a man named Baker. Seeking to become a person worthy of her attention, Billy took up the sport. This romantic drive is central to his actions on the court, transforming his matches into a stage to prove his worth and passion.

Personality-wise, Billy embodies the spirit of a cowboy, described as full of heart and possessing a wild, unrestrained nature. He maintains a distinctive appearance, typically wearing a cowboy hat over his brown hair, which sets him apart from other more conventional players. He is a right-handed player.

Within the story, Billy arrives in Japan ahead of the rest of his team alongside his teammate Kevin Smith. His most prominent role comes during a doubles match, where he partners with Michael Lee. They face the formidable Japanese pair of Keigo Atobe and Genichiro Sanada in the Doubles 2 match. During this intense match, his unconventional techniques prove effective. He successfully uses his Shotgun Volley to disrupt the rhythm and pace of the skilled Keigo Atobe.

Despite his spirited performance, his team ultimately loses the match. The turning point comes when his opponents execute their combination technique known as the Tango to Ruin, which elevates their teamwork beyond what Billy and Michael Lee could counter. However, the defeat does not diminish his personal triumph. Moved by his passionate and earnest play, Stephanie is impressed, and Billy successfully secures a promise from her to teach her tennis, achieving his personal goal.

Billy Cassidy is known for integrating movements from his rodeo and ranching background into his tennis techniques. His notable abilities include the Bull Rope Serve or Lasso Serve, a unique serve where the motion mimics a cowboy throwing a lasso or roping a bull. Another signature move is the Shotgun Volley, a powerful and impactful net play described as being strong enough to break up an opponent's established strategy. Through his background, motivation, and unique skill set, Billy represents a distinct blend of Western Americana within the series, using tennis not just for competition, but as a means to win the heart of the girl he admires.