Number

"'All 'Numbers'' numbers are not chosen randomly but are decided carefully with attention to details that can make the fans enjoy it in various places.' — Satoshi Kuwabara to ZEXAL followers in Twitter Segment."

"Number", pluralized as "Numbers" ( Nanbāzu) in Japan, is an archetype of prominent Xyz Monsters that play a major role of the story in the Yu-Gi-Oh! ZEXAL anime and manga series. Three variants of Number cards exist: "Emperor's Key Numbers" have numerical values equal to or lower than 100, while "Over-Hundred Numbers" have numerical values of over 100 and "Legendary Number" which are part of the Emperor Key Number but are unique.

So far, 72 of Numbers have been revealed, 46 out of those are known by name plus 6 Chaos versions.

Anime
When Yuma Tsukumo unlocked the door with his Emperor's Key, he opened a portal between the Astral World and his own, which allowed Astral to come into his world. Yuma found himself trapped within a sphere of blank cards, which shortly flew away. By crossing into Yuma's world, Astral's memories were transformed into the 99 "Number" cards and scattered in unknown directions (Number 39 stayed with him though). In order to regain his lost memories, Astral cooperates with Yuma. When Astral collects all of the cards, something else will happen besides his memory recovery. Astral thinks it will cause something troublesome, while Dr. Faker claims Astral will obtain a great power and use it for taking revenge on him.

Usually, "Numbers" are blank cards that take the form of what whoever finds them desires the most, design themselves to fit their original owner's Decks, and keep that form forever. Some Duelists may have more than one "Number" card that fit their Deck. Their card text is written in a strange alien language that only those who wield them can understand (in the dub, all "Numbers" appear normally like other cards seen with the exception of episode 2 which Number 39: Utopia is blurred). When they are Summoned, their respective numbers are printed on their users, appearing in such places as the forehead, neck, left and right hands (in Cameron Clix's case, it appeared under his right eye and in Number 56's Holder's case, it appeared in his left shoulder). When Summoned, "Numbers" take on what appears to be a sealed form, and when the "Number" starts to transform into their artwork, the "Number" then appears. Also, They use a galaxy-like "Overlay Network" portal in contrast to regular Xyz Monsters, who use a red spiral portal.

Astral described that there are 99 "Numbers", but according to Kite Tenjo, there are 100 "Numbers". As Astral already had 1 of the "Numbers", "Utopia", it is possible Astral was referring to the other 99 "Numbers". However, Dr. Faker believes that, aside from 99 "Numbers" which are Astral's pieces of memory, Astral himself is "the Original Number" (ナンバーズのオリジナル, Nanbāzu no Orijinaru). On the other hand, the manga is obvious about their number. When Astral obtained a memory piece from "Number 17", he said his memory is divided into 100 "Number" cards. This amount of number is later confirmed by both Kite Tenjo and Mr. Heartland.

When a "Number" is defeated, they are absorbed into Astral, and can be used by any Duelist whom Astral is currently with. Each "Number" contains a piece of Astral's memory, which he recovers each time he absorbs one. After Astral absorbs a "Number", they are stored in a Different Dimension Airship within the Emperor's Key. Each has its own pillar within the structure, engraved with its number.

Numbers have been shown to be very dangerous, as they are able to easily amplify the darkness and desires of their user, whether these cause good or bad. And easily possess their holder by their power, with the exception of Yuma due to the Emperor's Key, Kite Tenjo's Photon Transformation, and the Vetrix Family's crests. However, the power of Photon Transformation and the crests seem to be limited, and has fatal consequences and being used to wield a "Number" card will drastically exhaust their user.

As Number holders have a high level of a certain emotion, Chaos Numbers start to appear. For example, Astral and Yuma obtained "Utopia Ray" after Astral was happy to know Yuma considered him a friend. Reginald Kastle also obtained "Shark Drake Veiss" after his desire for revenge, brought on by "Shark Drake", overpowered him.

In Barian Invasion arc, "Over-Hundred Numbers" are introduced as the signature cards of the Seven Barian Emperors, which are ranked higher than 100. Astral cannot absorb them, as they are not part of his memories. Later on, the "Legendary Numbers" were introduced by Vector and Don Thousand that are located in ruins across the world, protected by guardians. Each of these "Numbers" corresponds to a legend inscribed on the walls of the ruins, telling the story of the person who occupied that location in a previous era, before they became ruins due to the ravages of time. Rather than take their form based on their holders Deck, they take a form relative to the legend that is described.

Original
Each of the monsters that are stated to have Astral's memories.

Legendary Number
Legendary Numbers are a group of special Number card that are located in Number's Ruins that are located in dimension and are required to find the Numeron Code.

Over-Hundred Number
Over-Hundred Numbers are signature "Number" cards of the Seven Barian Emperors, whose numbering go over the limit of 100 mentioned in the first season.

Number C (Chaos Number)
"Number C" (read as Chaos Number) are upgraded versions of the base "Number", which are Summoned by overlaying their base counterpart through Chaos Xyz Evolution and Rank-Up-Magic cards.

According to Luna, "Numbers" are mirrors that reflect the hearts of the people, and at that Shark's time, his heart was full of darkness, but Yuma and Astral freed him from that darkness.

Trivia

 * The story of the "Numbers'" origins is similar to the story of Pandora's box, where Pandora opened a box (a jar in some sources) out of curiosity and released its contents into the world, these being all the evils in existence which plague mankind. When she closed the box, the only thing left to her was hope. This is analogous to the "Numbers" being released into the world upon Astral's arrival, causing mayhem to whoever finds them and raising conflict among people, while the only one left for Yuma and Astral to battle the "Numbers" is called Hope.