Just to answer that question: You'll be able to try out the Dryad for FREE in a "Normalized" mode of the Terraria map as many times as you want. Then if you want to add her to your deck permanently, you can unlock her via Defender Medals (Our in-game earned currency) or Gems (Our premium currency). So you can keep her via gameplay or purchase.
Protip: We do events every weekend to help you get Defender Medals much faster, so definitely play on weekends!