Joseph King, known in Japan as Koujirou Genda, is a prominent character recognized as the goalkeeper for the prestigious Teikoku Academy, also referred to as Royal Academy in some adaptations. He is widely known by his epithet, the King of Goalkeepers, a title that reflects his immense skill and confidence in protecting the goal.

Physically, Joseph is distinguished by his tall stature and his spiky, brown hair, which becomes longer and more unkempt resembling a lion's mane in later story arcs. A notable feature is the pair of orange, tattoo-like markings under his eyes, which are a constant part of his design. In the second season, he also gains a scar over his right eye, marking a physical change that coincides with his character development.

His personality is a core part of his identity. Initially, Joseph is portrayed as a confident and somewhat arrogant player, a natural result of being the last line of defense for a dominant team. He carries himself with a serious and calm demeanor but possesses a strong sense of pride that makes accepting defeat very difficult for him. However, this pride is not his only defining trait. Beneath his stoic exterior, he is fundamentally a good person capable of forming strong loyalties, particularly to his teammate and friend, Jude Sharp. During his first appearance, when the team confronts their manipulative coach, Joseph is among the players who turn against him, demonstrating his courage and moral compass. He also has a protective side, once showing more concern for Jude's well-being than his own.

Joseph's primary motivation throughout the series is the pursuit of strength. After suffering a humiliating and physically devastating loss to the Zeus team, which left him and his teammates hospitalized, he is driven by a desire to become more powerful. This desire leads him to rejoin his former coach in season two as a member of Shin Teikoku. Here, his commitment to strength is tested as he uses a forbidden technique that harms his own body, showing a willingness to sacrifice for power. Ultimately, this path proves to be a mistake, and he is redeemed by recognizing his error and rejecting the manipulative control he was under.

His role in the story is primarily that of a formidable rival to the main character, Mark Evans. As the goalkeeper of Teikoku and later Neo Japan, he serves as a significant obstacle that the protagonists must overcome, representing the pinnacle of defensive skill. After his redemption arc, his relationships solidify, especially with Jude Sharp and his fellow defender David Samford. His journey from an arrogant rival to a respected ally showcases a clear character development, where he eventually accepts defeat with grace and uses it as fuel to improve rather than as a reason to fall into corruption.

In combat, Joseph is defined by his powerful blocking hissatsus. His early signature move is the Power Shield, a technique that creates an energy barrier to deflect shots. In his pursuit of greater power, he adopts the forbidden Beast Fang, a dangerous move that uses a massive, lion-shaped construct to swallow the ball. He later masters a refined version called High Beast Fang. By the third season, he also demonstrates the use of Drill Smasher and the combined technique Infinite Wall, showing his continuous evolution as a goalkeeper. His playing style is aggressive and dominant, designed to intimidate forwards and leave no angle for a shot on goal.