The Wild Card is an awesome way of getting the exact card you’re missing to complete a set!
There are 3 types of Wild Cards: 4* rarity, 5* rarity, and Ace Wild card. The rarity of the Wild Card indicates the highest rarity of the card it will be able to replace: a 5* rarity Wild card can replace any one missing card in your album, as long as it is of a 5* rarity or below.
You can get a wild card by completing special challenges and Quests! Be on the lookout for when they are offered as a reward!
Please note, once you get a wild card, it must be used to replace a card before it expires (you can see when this will happen by pressing the wild card icon at the top left corner inside the Album feature).
Good luck and have fun!