Mythology and the Signers:
Much like the original series, 5Ds is based on an ancient mythos which inevitably lends its power to the current generation core cast. In this particular case the focus is on the Nazca lines
which are seen throughout the show, on the title cards, and worked into the clothing of some of the characters. As the show progresses, Godwin reveals to Jack the existence and lore of the Signers - five chosen duelists who each bear a Mark of the Crimson Dragon. Jack was born with the Crimson Dragon's wings on his right forearm. Tenzen Yanagi reveals the same lore to Yusei who was chosen during his rematch with Jack by the People of the Stars to bear the Crimson Dragon's tail (also on his forearm) - which appears on his arm when he encounters another Signer. Aki Izayoi/Akiza Izinski and Luca/Luna each bear a mark of the Crimson Dragon's claws. The fifth/original Signer was revealed to be Rudger Godwin, possessing the Crimson Dragon's Head. Following Rex Godwin's bid for godhood the Marks shuffle and Crow becomes the new fifth Signer. Finally, Lua/Leo has achieved his goal of being a Signer, and given a new, sixth Heart dragon mark, after Aporia killed him in their duel and his revival by Ancient Fairy Dragon
Dark Signers:
The first true enemies to the Signers. Dark Signers are unfortunate duelists who, on the brink of physical death, swear revenge against another individual. Their souls then become host to an Earthbound Immortal (Jibakushin) and they become living corpses bent on destruction. The Dark Signers each wield an individual God card that, to summon, requires mass human sacrifice. True Dark Signers are distinguished by their small pupils and black sclera as well as the purple geoglyphs on their right arms in the shape of their individual God: one of the major figures of the Nazca Lines (the Spider, the Monkey, the Lizard, the "Spaceman/Astronaut" Giant, the Hummingbird, the Killer Whale, and the Condor).
Yliaster:
First mentioned by Rex Godwin early in the series, they are an ancient group of people focused on correcting history for the greater good, no matter what the cost. In the first season, they aided Godwin in the search for the Signers. In the second season, its confirmed that Rudger Godwin was pushed into becoming a Dark Signer by an unnamed Yliaster member who told him to look into the light of the Momentum to see the battle between the Crimson Dragon and Earthbound Gods. In the third season, three members: Jose, Placido and Luciano appear and call themselves "The Three Emperors of Yliaster". They exhibit the ability to alter the future to small extents (allowing them to enter the World Riding Grand Prix by taking the place of another group) and claim they are led by a God. Jose, Luciano and Placido were shown to be from the same man, Aporia. He, Bruno/Antinomy and Paradox were all robots created by ZONE (the true leader of Yliaster) in the future and sent into the past to correct the ruined future from happening. ZONE created the robots in likeness of his friends, who all died of old age in the apocalypse future. Finally Yliaster appears to be dissolved by the end of the series, with all of its members either dead or having left the organization.
Social Commentary:
5Ds takes place in a stratified society based on a modified caste system determined by birth. This results in the worst on-screen treatment of a Yu-Gi-Oh protagonist to date. Because he is Satellite-born, Yusei is tagged, jailed and generally discriminated against. Jack is also Satellite-born but hasn't suffered the same discrimination and mistrust as Yusei because Godwin has kept his origins a secret from the public. Godwin uses this to his advantage and threatens Jack's reputation as a way of keeping him obedient. However at the end of the Dark Signer Arc, the social divide has weakened considerably thanks to the main cast's efforts to reunite Neo Domino and Satellite.
Terminology:
In the tradition of Yu-Gi-Oh! GX, 5Ds is an Oddly Named Sequel. Rumors were abound concerning what "5Ds" actually stands for, such as the odd mis-named titles "Five Duel Scoop" and "Five Dimensions". Episode 96 confirms that it is a contraction of "five dragons", in reference to the five Signers and their dragons, which are needed to summon the Crimson Dragon.
