Signer

The Signers are the only group that can summon the legendary Crimson Dragon, the embodiment of the red star. There are six people in the world that have the Mark of the Dragon and Signer Dragons that fit into the legend. Each birthmark represents a part of the Crimson Dragon, as these marks themselves are actually broken pieces of the Crimson Dragon itself, that later were divided into six parts. Thus engraved and sealed into six people called "Signers".

The present day Signers in Yu-Gi-Oh! 5D's include Yusei Fudo, Jack Atlas, Crow Hogan, Akiza Izinski, Luna and Leo. Although in the dub Rex Goodwin has stated he is the fifth Signer, this contradicts the original anime until later on, when he infuses Roman's arm onto himself thus only technically making him the fifth Signer for the time by proxy. It is known that their powers were awakened by Rex Goodwin himself through the Fortune Cup. According to Goodwin, he did it so that they would defeat their archenemies, the "Dark Signers", along with the task of closing the door to the "Netherworld".

Although according to the Star Guardian, to those who are Signers exist within some kind of "fate", derived from the Dragon Star, one which cannot be escaped from. That strange fate was the one responsible for the fact that the four signers came together. Thus the "Fate of the Signers" unites each and every one of Yusei, Jack, Akiza, and Luna's own destiny together in order to guide them all. Nonetheless, according to Goodwin the four of them are not only duelists chosen by the Dragon Star himself, but they are also the reincarnations of the ancient Signers themselves. Also, it was already predicted thousands of years ago the four of them would encounter one another.

History
Before "Black-Winged Dragon" was revealed as a Signer Dragon, "Life Stream Dragon" was shown participating in the ancient battle between the Crimson Dragon and the Earthbound Immortals. It is unknown what exactly happened to Black-Winged Dragon after the battle against the Crimson Devil, Red Nova and same goes for Life Stream Dragon after the battle against the Earthbound Immortals before it was encased in armor to form, "Power Tool Dragon".

Jack Atlas' mark represents the Wings, Yusei's was the Tail, Luna and Akiza's marks each represent one of the Dragon's Claws, and Roman's was the Head mark. However, during the ultimate Shadow Turbo Duel against Rex Goodwin, the Tail and Head birthmarks are transferred: the Dragon Head is now under the possession of Yusei Fudo while Dragon's Tail (that was formerly Yusei's) choosing Crow Hogan as its new owner, making him the new fifth Signer (the head mark signifies a leader of sorts, choosing Yusei as the new one). Ultimately, a new mark, the Dragon Heart (seen on the full Dragon mark in the claws of the Crimson Dragon), was bestowed to Leo, making him the new sixth Signer.

It is interesting to note that both Jack and Roman had their Marks from birth while Akiza got hers at a young age (Luna received her Mark after she went to the Spirit World the first time) and Yusei Fudo did not receive his Mark until Episode 5 and it was not permanently burnt in until Episode 25. Crow received his Mark in Episode 64 and Leo, the last Signer, received his Mark in Episode 142.

Unlike their counterparts, the Dark Signers, whose Mark of the Shadows and Earthbound Immortals are connected, a Signer's Mark of the Dragon and Signer Dragon are connected only to the Signer and not to each other, as shown when Yusei inherited the Head Mark but kept Stardust Dragon.

A Signer's mark throbs and glows while they are in close proximity of one of the Dragons when it is fighting with a great deal of determination, particularly for the sake of their Signer. A Signer's Mark glows and painfully digs into their arm, when they are in the presence of a Dark Signer.

The Signer's mark also glows while in the company of the Team Ragnarok.

Trivia

 * The Heart Mark of the Crimson Dragon was actually seen before on the Yu-Gi-Oh! 5D's World Championship 2010: Reverse of Arcadia game when Rex Goodwin was showing the player and the real Signers the temple under his mansion. There was another detail on the front claw mark which looks like Leo's new mark.
 * The name of all Signer Dragons contain 2 adjectives, then 'Dragon' (including Red Dragon Archfiend - originally Red Demon's Dragon but excluding Stardust Dragon unless you count Stardust as two words).
 * Katsumi Ono revealed on Twitter that Leo becoming a Signer was planned from the beginning.