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.
Signature Monster Examples:
~Dark Magician - Yugi
~Blue-Eyes White Dragon - Kaiba
~Red-Eyes Black Dragon - Joey
~Insect Queen - Weevil
~Stardust Dragon - Yusei
Yu-Gi-Oh! : Ground Zero
Every dreamed of becoming the greatest duelist after Yugi Muto? Jaden Yuki? Or even Yusei Fudo? Here is your chance! [Always Accepting]