Other known variants
x2Concealed Cards
You must discard an Energy card from your hand in order to use this Ability. Once during your turn, you may draw 2 cards.
Moonlight ShurikenDiscard 2 Energy from this Pokémon. This attack does 90 damage to 2 of your opponent's Pokémon. (Don't apply Weakness and Resistance for Benched Pokémon.)
It appears and vanishes with a ninja's grace. It toys with its enemies using swift movements, while slicing them with throwing stars of sharpest water.
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.
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.
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.