Got the Card "Best Friend Forever" as a treasure. The card text states that it lasts FOREVER. What it means is that even after your run, it will be in all subsequent runs as well. It also combines with other friends in your deck.
This brawl also has a different feature as opposed to the other dungeon runs, which is the possibility of "Empty Card Buckets", meaning you don't have to dilute your deck after a boss with 3 more cards.