Story & World
Kardia
The active leader of your three characters in a battle. The Kardia grants the team an exclusive effect (a Law / Exclusive Principle) and can be moved and controlled during the fight. (Some sources use Kardia for any playable character, to confirm at the beta.)
Kardia is the game's term for the active leader among the three Animasters you bring into battle. The Kardia grants the team its Exclusive Principle and can be moved and controlled during the fight, while all three characters contribute Offer Relics.
Some sources use "Kardia" more broadly, for any playable or obtainable character as opposed to an NPC, and that's how our characters database labels them. Which reading is the official one should get settled at the beta.
In practice, pick a Kardia whose Principle fits the board you're building, since the effect is team-wide. The Handbook adds a practical note: you can save several team setups to swap between, and on PC a right-click quickly deploys or equips your Anima, Kardia and Relics.