- You can play Cynthia & Caitlin as long as there's a Supporter card in your discard pile. You can also play it by discarding a card from your hand if there's at least 1 card in your deck. But if you have no cards in your deck, you cannot discard a card from your hand.
Source:
Cosmic Eclipse FAQ (2019-10-18), TPCi Rules Team (2019-10-31)