Trainers » Cynthia & Caitlin
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)

Relevant cards

Note: All applicable cards may not be listed below.
SM12_189
SM12_189
SM12_228
SM12_228