You actually can build your deck! There is a telemart in your TV where you can buy cards for money from defeated enemies. We were unable to think of a way to tell the player that there is a shop, although we mention it at the end of battle and in the dialogue with cat, even we know it was not enough, but time was running low. Also, interacting with objects in house gives you cards for your deck, but somewhere down the pipeline, the dialogue phrase that should play when you find a card stopped playing, despite cards being given ¯\_(ツ)_/¯. Your deck should have at least 10-12 cards in order for you to be able to move them and properly deckbuild.