Ken Masters is a central character in the Street Fighter series, known primarily as the best friend and friendly rival of the series' main protagonist, Ryu. Born into immense wealth as the son of a hotel tycoon, Ken's background stands in stark contrast to the ascetic life of his training partner. When he was approximately twelve years old, his father, fearing his son would become a spoiled brat, sent Ken to Japan to learn discipline by training in the Ansatsuken martial arts style under the master Gouken, a friend of the family. This journey placed him alongside Gouken's adoptive son, Ryu, with whom he initially had a mischievous relationship, playing pranks to get a reaction out of the serious boy. This dynamic eventually forged a deep, lifelong bond of mutual respect and brotherhood.

Ken's personality is a direct foil to Ryu's stoic and serious demeanor. He is characterized as flashy, confident, unorthodox, and even arrogant or short-tempered at times. However, beneath this brash exterior is a good-hearted and easygoing individual who respects his opponents and never backs down from a challenge. While Ryu is consumed by his quest for self-improvement and mastery of his art, Ken's motivations have always been more grounded. After completing his training at age twenty-three, he returned to the United States and enjoyed a successful career as a martial arts champion, winning most tournaments he entered and earning the title of America's strongest man. His life took a significant turn when he met a woman named Eliza, who would become his girlfriend and later his wife.

The tragic death of their master Gouken at the hands of Akuma is a pivotal event in Ken's story, sending him on a journey to find Ryu and inform him of the loss. In a key moment of support, Ken gave his own red headband to a dejected Ryu to help him focus, an accessory that would become iconic to Ryu's design from that point forward. Over time, Ken's role in the story evolved beyond that of a simple rival. His relationship with Eliza deepened, and they eventually married, with Ken famously vowing only to do so after achieving a decisive victory over his friend Ryu. Starting a family, including the birth of his son Mel, became his primary motivation, fundamentally changing his outlook on life. During the events of Street Fighter IV, he hesitated to enter a tournament because Eliza was heavily pregnant, only competing after she encouraged him to go, and during his fights, he frequently demonstrated his concern for her wellbeing.

Ken's key relationships define his character arc. His friendship and rivalry with Ryu is the cornerstone of the series, representing two different paths in life: one of total dedication to the fight and one that balances fighting with family responsibilities. He also has a close relationship with Eliza, and through her he becomes the brother-in-law to the American Air Force officer Guile, who is married to Eliza's sister. His success in life, with a loving family and immense wealth, contrasts sharply with Ryu's wandering, penniless existence, but Ken deeply values the purity of his friend's path. In later games, he takes on the role of a master himself, reluctantly training a Brazilian teenager named Sean Matsuda and also beginning to teach his own son, Mel, the basics of their fighting style.

In terms of abilities, Ken is a master of the Ansatsuken style, sharing many techniques with Ryu. However, their applications differ significantly to reflect their personalities. While Ryu's moves are focused, powerful, and precise, Ken's style is more aggressive, flashy, and suited for close-quarters combat. Ken has particularly mastered the Shoryuken, or Dragon Punch, an uppercut technique that he can imbue with flames, making it incredibly powerful and destructive. His Tatsumaki Senpukyaku hurricane kick is also distinct, typically delivering multiple hits in rapid succession. Conversely, his Hadouken energy projectile is often less focused than Ryu's, making it weaker or slower, as he prioritizes rushing down his opponent over maintaining distance. He is noted for his incredible durability and tenacity, having continued to fight even after sustaining severe injuries, such as against the assassin Vega while losing a significant amount of blood. Over the course of the series, Ken's journey has been one of development from a potentially spoiled, arrogant rich kid into a disciplined fighter, a devoted husband and father, and a mentor, proving that one can be a formidable warrior while still embracing life's personal joys and responsibilities.