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, Yu-Gi-Oh! ZEXAL and Yu-Gi-Oh! VRAINS, they tend to be Synchro Monsters, Xyz Monsters and Link Monsters, respectively. 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

 * While the protagonist of Yu-Gi-Oh! ZEXAL has the same ace monster ("Number 39: Utopia") in the anime and manga, a different Xyz Evolution method is used for said ace monster in each version: Chaos in the anime and Shining in the manga.