What exactly is a Warrior of Light, and furthermore, how many are there?

Is a Warrior of Light someone in communication with Hydaelyn? Would that then include Minfilia and Iceheart, or s it someone with only a specific blessing?

Are the Warriors of Light from before the Calamity the same thing as our current player characters? Do any of them still technically live?

Lastly, is there really only one Post-Calamity Warrior of Light?