I protect the sheep in our society from the wolves.
The gist of putting a card into a check is "one card type per player per check," within the limits that cards have on their text. The revised rulebook (available on the Paizo website) clarifies this in much greater detail.
So I could discard a Blessing for an extra die of whatever type of check I am doing, and I could also use a Weapon to add another die, as well as an Item and Spell if they all allowed me to add a die to the certain type of check I was doing? All damage I take from monsters/henchmen makes me discard cards from my hand regardless of what the damage is called on the card (acidic, electrical)? With 2 characters in play can each of them discard a Blessing of the Gods to each add a die to the Combat Check?
If I decide to start using a different character, does that character just start with their own 15 cards?