Mr. X, also known as Mister X, is the boss of the Tiger's Den stable in Tiger Mask W. He is the main antagonist of the original Tiger Mask franchise and serves as an overarching antagonist in this sequel series, which is set several decades after the original. His appearance remains consistent with the original character: he is a male adult with short blue hair and blue eyes, typically depicted wearing refined formal attire that includes a top hat, a monocle, and a cane, which are relics of his earlier incarnation. In the Tiger Mask W timeline, these items are kept in his office as relics of his predecessor, suggesting that the Mr. X seen in this series may be a successor who is identical in appearance, clothing, and character to the original.

Mr. X is portrayed as a cunning, secretive, and manipulative figure who wields considerable power and influence behind the scenes. He is driven by a deep-seated grudge against the original Tiger Mask, who betrayed the Tiger's Den and became a hero wrestler, and this animosity extends to the new Tiger Mask, Naoto Azuma, whom he regards as the continuation of that betrayal. His primary motivation is the destruction of the new Tiger Mask and the preservation of the Tiger's Den's control over the professional wrestling world. He seeks to crush any opposition to the organization's dominance, using both legal and illegal means to achieve his ends. He is willing to go to any lengths to ensure the success of his schemes and is not easily swayed by emotion or sentiment, making him a formidable and calculating adversary.

In the story, Mr. X operates as the high-ranking leader of the Tiger's Den, the shadowy organization that trains villainous wrestlers. He oversees the Global Wrestling Monopoly, a rival American wrestling federation secretly controlled by the Tiger's Den, and works alongside his subordinate Miss X, who handles much of the day-to-day management of the organization. He is responsible for orchestrating many of the schemes that target the new Tiger Mask and his allies, directing the actions of the GWM's top wrestlers, including Tiger the Third and Yellow Devil. He views the new Tiger Mask as a personal threat and a symbol of the original's treachery, and he is determined to see him unmasked and defeated. His presence looms over the conflict, as he pulls the strings from the shadows, manipulating events to wear down his enemies.

Key relationships include his partnership with Miss X, who serves as a manager and enforcer under his leadership. He also has a direct adversarial connection to the new Tiger Mask, whose very existence reopens old wounds from the past. His authority extends over the GWM's top wrestlers, such as Tiger the Third, though the latter sometimes operates with a degree of autonomy that Mr. X cannot fully control. His hatred for the original Tiger Mask colors his every action, and he carries the memory of that betrayal as a driving force throughout the series. The character does not undergo significant personal development or redemption; he remains a static antagonist whose goal is to destroy the new Tiger Mask and reassert the Tiger's Den's dominance. His notable abilities lie not in direct physical combat, which he rarely engages in, but in his strategic acumen, his vast resources, his ability to manipulate events and people, and his mastery of underhanded tactics, including the development and deployment of brutal wrestling techniques designed to injure or kill opponents. He is a master of psychological warfare and organizational control, using his wealth and influence to maintain his grip on the world of professional wrestling.