you can evolve cards when have many similar cards in your account.
When you have 2 similar cards, you can evolve them into their EX form. Common to common EX; rare to rare EX; or Ultra to Ultra EX. All the cards can be evolved only once except some special reward cards when you invite others to join the game which can evolve from 1* to Ultra Rare Ex (6*)
To evolve, normally you need to consider two things:
First, do you evolve them to fill your gallery. If so, click ‘evolve’ and choose the cards, pay some processing fee, then ta da, done.
If you evolve the card because it is your main front line and you want to make it stronger. Then you should consider this: evolve a card will reset all the skill back to level 1, and retain 5% of the status if the card is not max before evolve, 10% if the status is max. So, very straight forward, before you evolve the card, try to use any common card to enhance both the cards (you need two cards to evolve) up to their maximum status. Then only evolve them to get a perfect level 1 EX status, as the skill level will be reset, it will always be 1 after evolve.