Signature card

A signature card is a card most associated with a Duelist, and tends to be used in a majority of their Duels. Duelists may have more than one signature card. They might have a connection and the Duelist may value it. Signature cards tend to be Monster Cards and to have alternate artworks in the OCG and TCG. In some cases, signature cards are created by their owners and/or only one copy of them exist in the anime like "Elemental Hero Neos" and the "Numbers".

In Yu-Gi-Oh! 5D's, they tend to be Synchro Monsters and in Yu-Gi-Oh! ZEXAL, they tend to be Xyz Monsters. Signature Monster Cards may also be called Ace Monsters.

Some video games have a similar concept known as a Key Card, in which the player selects a certain card as their "ace". Yu-Gi-Oh! The Duelists of the Roses has "Deck Leaders".

Trivia

 * The ace monsters of every main protagonist share the same amount of ATK: 2500.
 * With the exception of "Dark Magician" and "Firewall Dragon", the Types of every main protagonist's ace monster is either Dragon ("Stardust Dragon" and "Odd-Eyes Pendulum Dragon") or Warrior ("Elemental HERO Neos" and "Number 39: Utopia") and they share the same amount of DEF: 2000.
 * With the exception of "Stardust Dragon", the Attributes of every main protagonist's ace monster is either LIGHT ("Elemental HERO Neos", "Number 39: Utopia", and "Firewall Dragon") or DARK ("Dark Magician" and "Odd-Eyes Pendulum Dragon").
 * The ace monsters of every main protagonist's rival share the same amount of ATK: 3000.
 * With the exception of "D/D/D Doom King Armageddon", every rival's ace monster is a Dragon-Type and they share the same amount of DEF: either 2000 ("Armed Dragon LV10" and "Red Dragon Archfiend") or 2500 ("Blue-Eyes White Dragon" and "Galaxy-Eyes Photon Dragon") except Borreload Dragon (Since Link monster doesn't have DEF)
 * With the exception of "Armed Dragon LV10", the Attributes of every rival's ace monster is either LIGHT ("Blue-Eyes White Dragon" and "Galaxy-Eyes Photon Dragon") or DARK ("Red Dragon Archfiend", "D/D/D Doom King Armageddon", "Borreload Dragon").


 * The ace monsters of every manga counterpart of main protagonist share the same amount of ATK: 2500.
 * Like its anime counterpart, with the exception of "Dark Magician", the Types of every main protagonist's ace monster is either Dragon ("Stardust Spark Dragon" and "Odd-Eyes Phantom Dragon") or Warrior ("Elemental HERO Terra Firma" and "Number 39: Utopia") and they share the same amount of DEF: 2000.
 * With the exception of "Elemental HERO Terra Firma", the Attributes of every main protagonist's ace monster is either LIGHT ("Stardust Spark Dragon",and "Number 39: Utopia") or DARK ("Dark Magician" and "Odd-Eyes Phantom Dragon").
 * Unlike their anime counterpart,the manga counterpart ace monsters of every main protagonist's rival share varies amount of ATK: Either 2800("Light and Darkness Dragon" and "D/D/D Supersight King Zero Maxwell") or 3000("Blue-Eyes White Dragon","Galaxy-Eyes Photon Dragon" and "Hot Red Dragon Archfiend").
 * Anime and Manga of Yu-Gi-Oh! ZEXAL protagonist and rival shares the same ace monster ("Number 39:Utopia" and "Galaxy-Eyes Photon Dragon") but they have different evolution (Chaos Xyz Evolution in anime and Shining Xyz Evolution in manga)