Disappeared Dad: Daigo Fukami is Makoto and Kanon's father, who abandoned his children after his wife's death.
Evil Counterpart: Argos to Takeru Tenkuji, as both are died and revived by Hermit/Edith and granted with the powers of Kamen Rider Ghost.
Dark Ghost to Kamen Rider Ghost. His Dark Damashii appearance is analogous to Ghost's Ore and Toucon Boost Damashii.
Horned Humanoid: Extremer has a gigantic horn on his helmet, bigger than any of the other Riders'!
Instead of Ghost's whispy single horn and Specter's two curved horns, Zero Specter has three horns.
Long-Lost Relative: Argos is Adonis' oldest child, and as such he is the older brother of Alia, Adel and Alain.
Meaningful Name: Argos' name refers to Hera's watchman in Greek mythology, the many-eyed giant Argos (or Argus). Hera honored the giant postmortem by preserving his hundred eyes in the tails of peacocks which Argos alludes to with his goal of collecting 100 Eyecons to create Kamen Rider Extremer, whose Eyecon laden wings are designed to resemble the tail of a peacock.
Obviously Evil: Dark Ghost's Driver announcement, "Yami no chikara! Warui yatsura!" literally comes out to "The power of darkness! The bad guy!" It doesn't even hide Argos' alignment as it's declaration isn't even "Dark Ghost" but instead "Dark Rider". (Near the end of the series, when the sage uses the Dark Ghost suit, that part's skipped.)
Purple Is Powerful: In contrast with Specter's blue, Zero Specter's Zero Damashii appearance consists of purple and black as his main colors.
