[@Bishop]

For the time being I think it's best if everyone in the party has just one character. 

You bring up an interesting idea, but it conflicts with the way Pact magic works in this world (which summoning typically falls under). In order to use Pact magic you have to swear an oath (or be forcibly bound) to one patron. They then provide you with magic through their own otherworldly power. Typically betraying that oath is a very bad choice for a character to make.

Another potential avenue is shapeshifting (a form of Primal magic) which could allow you to turn into certain creatures. I just want to make sure that all characters are balanced out here, and fit well into the lore of the world. Though if you want a character with a card motif there are some avenues we can explore (poison cards, cards as projectile weapons, etc.)  

Lastly, your character could actually be a Bard. Bard's sing or play instruments to aid in combat. So your character could potentially write hymns based on legends/heroes to grant themselves or allies abilities/buffs in combat. For example, a hymn/song about a mighty warrior may rally a downed teammate causing them to get back up and have increased strength. 

Also, if you have a number of ideas please feel free to share them here or through PMs! I think it'd be nice to hear constructive input from everyone regarding the characters that will end up in the main party!