Chairman Kagemaru is the main antagonist of the first season of the anime, serving as the founder and former chairman of Duel Academy as well as the secretive leader of the Shadow Riders. His name is derived from Japanese words where "Kage" means shadow and "maru" often denotes perfection or a common masculine ending, a fitting title for a character who operates from the shadows. Kagemaru is depicted as an elderly man of advanced age, said to be over one hundred years old, who is in such terrible physical health that he cannot stand or move without the aid of a mobile life-support system, a large mechanical apparatus that sustains him. He has a bald head with long, flowing white hair and a matching beard, giving him the frail appearance of a wise but weathered figure. However, this appearance is deceptive, as his true physicality is revealed when he temporarily regains his youth, transforming into an incredibly muscular and powerful man with long black hair, demonstrating that his aged state is a result of time rather than inherent weakness.

Kagemaru's primary motivation is the desperate pursuit of eternal youth and immortality. Plagued by his deteriorating condition, he seeks to regain the vitality and strength of his younger days, a desire that has consumed him to the point of orchestrating a massive conspiracy. His plan centers on the Sacred Beasts, three incredibly powerful cards he had previously sealed away. To revive them, he needs a vast amount of Duel Energy, the spiritual energy generated by dueling. To this end, he established Duel Academy itself, creating an institution designed to be a breeding ground for duelists whose battles would generate the necessary energy to fuel his ambition. He assembled the Shadow Riders, gifting them their Shadow Charms, and instructed them to challenge the key keepers of Duel Academy, not necessarily to win, but simply to duel, as the conflicts themselves served to weaken the seal on the Sacred Beasts. His ultimate goal, once the Sacred Beasts are revived, is to use their power to drain Duel Monster Spirits from across the world, absorbing that energy to restore his own youth and achieve a form of immortality, seeing the spirits and anyone who stands in his way as mere tools for his rejuvenation.

In terms of personality, Kagemaru embodies a combination of patient, ruthless calculation and desperate longing. In his aged, enfeebled state, he presents himself as a patient and manipulative mastermind, watching from the shadows and moving his pieces like a chess master without regard for their well-being, as he views the Shadow Riders as disposable pawns. He can be taunting and arrogant, as seen when he appears on a screen to gloat over Chancellor Sheppard after the loss of several Spirit Keys. Despite his age and apparent frailty, he possesses a deep-seated pride and a domineering will, confident in the power of the Sacred Beasts and his own plan. Upon achieving his youthful form, his personality shifts to become more unruly and boisterous, reveling in his regained strength and power, speaking with a newfound confidence and aggression that contrasts sharply with his earlier, more measured demeanor. His driving fallacy is the belief that youth and the power it brings are worth any sacrifice, a desire so strong that it blinds him to the morality of his actions until his defeat.

Kagemaru plays the role of the primary antagonist and final obstacle in the first story arc. He first appears as a disembodied voice instructing Professor Banner, revealing he has been monitoring Jaden Yuki for some time. After all seven Shadow Riders are defeated, he arrives at the Duel Academy heliport in his life-support capsule, revealing himself as the true mastermind. He ignores challenges from other top students and specifically demands to duel Jaden Yuki, whom he sees as the key to completing his plan. During this duel, he successfully summons all three Sacred Beasts Uria, Lord of Searing Flames, Hamon, Lord of Striking Thunder, and Raviel, Lord of Phantasms, and uses their power to absorb Duel Spirits, which successfully rejuvenates him into his muscular, youthful form. After his defeat by Jaden, the absorbed energy is released, and he reverts to his aged state. He immediately renounces his former ways and asks for forgiveness, a plea that Jaden happily grants, demonstrating a moment of complete reformation. He is last seen being taken to a hospital after Jaden accidentally injures him with a congratulatory bear hug. Following his redemption, Kagemaru returns in a later season as a supporting character, now confined to a wheelchair but no longer reliant on his life-support system, working alongside Sartorius to warn Jaden of a new impending darkness, and is present at the graduation ceremony, having fully atoned for his past actions.

Kagemaru's key relationships are defined by his overarching plan. He is the master of the Shadow Riders, whom he recruited and equipped, but he holds no loyalty to them, treating their sacrifices as necessary steps to generate Duel Energy. His most significant relationship is with the protagonist, Jaden Yuki. He sees Jaden not as an enemy but as the ideal opponent, the duelist whose spirit and energy are potent enough to finally awaken the Sacred Beasts and complete his rejuvenation, making their confrontation the culmination of his long-conceived scheme. His relationship with the faculty of Duel Academy, particularly Chancellor Sheppard, is one of deception, having used his position as founder to hide his true intentions from them while creating the school for his own selfish purposes.

Kagemaru is notable for his dueling abilities, specifically his specialized Sacred Beast Deck. He does not use a standard strategy but rather builds his entire deck around summoning and supporting the three Sacred Beasts, utilizing numerous Continuous Trap Cards to summon Uria and Continuous Spell Cards to summon Hamon, while using Fiend-type monsters to tribute for Raviel. He relies heavily on the field spell Fallen Paradise, which allows him to draw two cards per turn, compensating for the immense resource cost of summoning his gods. He demonstrates a deep understanding of Shadow Games and Shadow Charms, a knowledge gained from his long history with the Sacred Beasts, although he notably lacks the ability to see Duel Monster Spirits himself. In his rejuvenated state, he also displays incredible physical strength, casually tossing aside his heavy, mobile life-support system with one hand, a testament to the raw physical power he has regained. His ultimate development is his reformation; after being defeated and returning to his aged state, he abandons his goal of immortality, accepts his age, and chooses to live out his remaining days peacefully, having learned that youth regained through evil means is not worth the price.