Narutaki is a recurring antagonist in Kamen Rider Decade whose true origins are never fully explained. He presents himself as a prophet, though his background remains deliberately mysterious throughout the series. He possesses the ability to travel freely between the alternate reality worlds known as A.R. Worlds, moving through dimensional curtains just as the main characters do, and he appears to be immune to the dimensional convergence that threatens those worlds.

His defining motivation is a deep, unwavering conviction that Tsukasa Kadoya, the protagonist who transforms into Kamen Rider Decade, is a destroyer who will bring ruin to all worlds. Narutaki actively works to prevent this fate by warning other Kamen Riders about Decade, often appearing suddenly during battles to declare that Decade will awaken as a devil. He spreads rumors and incites Riders from different worlds to fight against Decade, hoping to eliminate the threat before it can fulfill its prophesied role. His catchphrase, which he frequently shouts at Tsukasa, encapsulates his intense hostility.

Narutaki's role in the story is that of a persistent, enigmatic foil to the protagonist. He serves as a constant reminder of the series' central question: whether Tsukasa is destined to be a hero or a destroyer. He does not directly confront Decade in personal combat most of the time, but instead uses his abilities to summon evil Riders from other dimensions to attack Decade. He also manipulates events behind the scenes, such as when he restores the villainous organization Dai-Shocker and later forms an alliance with Super Sentai villains in crossover films. Throughout the main series, he frequently approaches Natsumi Hikari, one of Tsukasa's companions, to warn her about the danger Decade represents, attempting to turn her against him. Despite his persistent opposition, he occasionally acts against greater threats, revealing that his primary goal is the protection of the worlds, not personal destruction.

Key relationships define his character. His most significant dynamic is with Tsukasa Kadoya, whom he obsessively tracks across every world, believing that Tsukasa's very existence is the cause of the multiverse's collapse. He also interacts repeatedly with Daiki Kaito, the interdimensional thief who becomes Kamen Rider Diend, viewing Daiki as another anomaly. His attempts to influence Natsumi Hikari create a tense relationship where she initially refuses his offers but later becomes disturbed by his warnings.

Narutaki's development is minimal, as his character remains largely static and mysterious. He begins and ends the series as an unexplained force of opposition. Later appearances in crossover films show him continuing his role as a secondary antagonist, sometimes allying with larger villain organizations when it serves his purpose. His identity, nature, and ultimate fate are never clarified, leaving him as one of the most enigmatic figures in the series.

His notable abilities include dimensional travel, allowing him to appear and disappear at will across different worlds. He can summon dark versions of Kamen Riders from other dimensions to fight on his behalf. In certain crossover films, he demonstrates the ability to transform into various forms, such as Colonoel Zol, Golden Wolfman, Doctor G, and Kani Laser, suggesting a shape-shifting or transformation capability. He also appears to possess a protective aura that makes him immune to the dimensional disruption affecting other characters. Despite his significant influence, he rarely engages in direct physical combat himself, preferring to act through proxies and manipulation.