Trading CardsTrading Cards are needed if you want to use the Battle Arena. There are many to collect and are available to buy from the Trading Card Shop in Undying Woods. Trading Cards can also be gained from Account Upgrades as well as Missions, Quests, Fairies, etc while playing on the site.
If you want to battle you must keep your Trading Cards in your Battledeck.
Once cards have been added to your deck they cannot be removed.
You will find a comprehensive guide to Trading Cards, Opponents and Battling at
gennie's Mara Battle Guide