Yor Universal Profile levels up as you acquire XP by playing the game. You also acquire tickets by completing missions. There is a Ticket Counter where you can redeem said tickets for unique cards. Currently there are 5 players in Counter available for purchase. The best card card you cannot purchase until you reach Level 20 on your Universal Profile.
There is a common misconception that leveling up will make more cards available. This is wrong. As time passes SSDS will make more cards available in the Ticket Counter that require a higher Level to unlock.