Cards are a feature which rewards players for fighting the same enemies more.

As you kill more of a single type of enemy, you will gain progress towards that enemies card. Each rank of a card provides a 0.5% boost to xp and drachma gains from that enemy, and they have a maximum rank of 99. This means with a maxed out card for an enemy you will get a 49.5% boost to xp and drachma gains from that enemy. This boost is multiplied with the base gains from that enemy (so a rank 99 card will mean you get 1.495x the amount of xp/drachma from that enemy).

As you rank up cards more, the requirements for the next card will increase. Once a card reaches rank 5 it is "unlocked" and will appear on your profile.

Cards are account based, which means that they are very helpful for lvling up multiple characters as the xp and drachma boost will still apply. Bounty kill requirements also scale with a cards rank, so if you have a lot of cards unlocked you will see that your bounties will become much harder to complete