Argos (アルゴス Arugosu), is the first born son of Adonis who died in an unknown illness many years ago before the events of the series where he serves as the main antagonist of Kamen Rider Ghost the Movie: The 100 Eyecons and Ghost's Fated Moment. Like Takeru, he resurfaces as a ghost and given a Kamen Rider power by Edith to transform into Kamen Rider Dark Ghost (仮面ライダーダークゴースト Kamen Raidā Dāku Gōsuto), as well as guidance by Daigo Fukami. Unfortunately, Argos' recent ambition became too dangerous, leading to his downfall and betrayal of both Edith and Daigo. His primary goal is now to collect the 100 heroic Eyecons to produce the ultimate Ghost Eyecon belt, the Extremer Driver (エクストリーマードライバー Ekusutorīmā Doraibā), which will allow him to transform the world into one filled with ghosts. He creates an Eyecon-shaped world above the Earth known as the Island of Eyecons (眼魂島 Aikon-jima?) where many historical people live with the intent of luring the Kamen Riders into his world and steal the other 15 heroic Eyecons from their possession so that he can enact his plan. His primary target is to capture his human world counterpart, Takeru Tenkūji, alive to complete the Extremer Driver, as Argos holds what appears to be Takeru's original human body, making him a perfect vessel for the belt. He also wants to persuade his brother Alain into helping him, but thanks to Takeru he has become a much kinder person. This disappoints Argos to the point where he tries to mercilessly kill his brother. Once Takeru is captured and all 100 heroic Eyecons are collected under Argos' possession, he finally dons the Extremer Driver and becomes the ultimately dangerous Kamen Rider Extremer (仮面ライダーエクストリーマー Kamen Rider Ekusutorīmā).
Prior to his downfall when he was alive, Argos used to be a kind prince towards his family and their people. During the final battle, Edith recreated a copy of Dark Ghost's power and used it to participate in the Kamen Riders' final confrontation against Adel but his old age forced Yurusen to withdraw him. Edith was briefly mistaken as Argos by Takeru in this form before he corrected the boy.
As Kamen Rider Dark Ghost, Argos uses the Ghost Driver and Dark Ghost Eyecon to access his primary form. Like his human world counterpart, he wields both the Gan Gun Saber and the Sunglasseslasher as his personal weapons. Also, like the other Riders, Dark Ghost is able to change into any known "Damashii" form based on the Eyecon he inserts into the Ghost Driver:
- Napoleon Damashii (ナポレオン魂 Naporeon Damashii) empowers Dark Ghost with the spirit of the French emperor. Dark Ghost captures Napoleon in the Island of Eyecons.
Argos is portrayed by Ryo Kimura (木村 了 Kimura Ryō). As a child, Argos is portrayed by Shōta Ishino (石野 湘太 Ishino Shōta).