Abilities » Concealed Cards
When using Radiant Greninja's "Concealed Cards" Ability, can I discard an Energy card from my hand if there are no cards left in my deck?
No, discarding an Energy card is a cost (as identified by the "in order to" phrase) so you can't do it if you can't do the actual effect of the card.
Source: TPCi Rules Team (2023-05-04)
Abilities » Concealed Cards
If my Radiant Greninja has already used its "Concealed Cards" Ability during my turn but then goes out of play, if I bring it back into play can I use "Concealed Cards" again on the same turn?
Yes, you can.
Source: TPCi Rules Team (2022-06-16)
Abilities » Concealed Cards
When using Radiant Greninja's "Concealed Cards" Ability, can I discard an Energy card from my hand, and then later during my turn draw 2 cards?
No, you must draw the 2 cards immediately after discarding the Energy card. "Once during your turn" means you cannot use this Radiant Greninja's Concealed Cards Ability multiple times during the same turn.
Source: Astral Radiance FAQ; TPCi Rules Team (2022-05-26)