In the universe of Kamen Rider Decade, the character known as Apollo Geist primarily operates under the human alias of Guy. He is a high-ranking executive and the Chief of Security for Dai-Shocker, a vast coalition of villainous organizations from across different realities. This version of Apollo Geist originates from an Alternate Reality known as the World of X-Rider, where he was originally a cyborg created and revived by the Government of Darkness (G.O.D.) after a previous defeat. However, his resurrection came at a severe cost, leaving him with a drastically shortened lifespan.

To combat his own mortality, Guy is forced to rely on a device called the Perfecter, which is embedded in the center of his face. This device allows him to sustain himself by siphoning the life force, or life flames, from human beings. In his daily life, he adopts the sophisticated appearance of a man in a white tuxedo and black gloves, and he is known to have a particular fondness for coffee. When engaging in combat, he cries out Apollo Change to transform into his more powerful masked cyborg form, donning a dark suit, a red medieval-style helmet, and a flowing white cape.

The personality of Guy, particularly in this iteration, is marked by arrogance and a pragmatic ruthlessness. He is depicted as a cowardly and snide villain who believes himself to be the greatest nuisance in the universe, lacking the sense of honor that some other antagonists might possess. He is willing to use any underhanded method to achieve his goals, including taking children hostage, and has a habit of underestimating his opponents, a flaw that ultimately contributes to his downfall. His primary motivation is survival and power. He actively works to hasten the convergence of the Nine Worlds, merging them into a single Rider War world to make it easier for Dai-Shocker to conquer, all while desperately seeking a permanent solution to his own decaying body.

Throughout the series, Guy plays the role of a primary antagonist, directly opposing the protagonist, Tsukasa Kadoya (Kamen Rider Decade). After his Perfecter is stolen by the rival rider Diend and subsequently destroyed by Decade, Guy is forced to find a new method of survival. He devises a plan to enter the World of Kiva and marry Yuki, the Queen of the Fangire race. This union grants him the powers of a Fangire, allowing him to evolve into a significantly more powerful form known as Super Apollo Geist. In this new state, his helmet features a distinctive blue stripe, and he gains the terrifying ability to erase entire alternate realities from existence, accelerating the fusion of worlds at will. His key relationships are largely transactional; he views others as tools for his survival, whether they are the monsters he commands or the queen he marries for power. He even attempts to kidnap Natsumi Hikari, a companion of Decade, to force her into becoming his bride after Yuki is destroyed.

In combat, Apollo Geist is an immensely powerful fighter, often more than a match for Decade or Diend in single combat. He wields an array of weapons, including his favored firearm, the Apollo Shot, and a thin sword known as the Apollo Fleur. His signature defensive and offensive tool is the Geist Cutter, a sturdy, circular shield with a saw-like edge that he can throw as a projectile. As Super Apollo Geist, his power multiplies; he can revive defeated monsters from across the Nine Worlds to fight for him and uses an upgraded Super Apollo Shot to unleash devastating attacks. His development arc is a desperate descent from a scheming executive clinging to life to a chaotic force of nature determined to drag all of reality down with him, culminating in a final battle where he is ultimately destroyed by Decade Complete Form and Diend.