Dryad

The Dryad is an NPC vendor that will spawn once the following criteria have been met:
 * There is an empty House.
 * You have killed one of the following bosses: Eye of Cthulhu, Eater of Worlds, Brain of Cthulhu or Skeletron.

She will tell you the percentage of Corruption vs. Hallow in the game world. If the world has more Hallow than Corruption, she will say "Keep up the good work!". If there is more Corruption than Hallow, and at least 20% corruption, she will say "Things are grim indeed.". Otherwise she will say "You should try harder."

Items sold
Once the Dryad has moved into a house, she will sell the following:

Names
The Dryad may have any of the following names: Alalia, Alura, Ariella, Caelia, Calista, Celestia, Chryseis, Emerenta, Elysia, Evvie, Faye, Felicitae, Lunette, Nata, Nissa, Tatiana, Rosalva, Shea, Tania, Isis and Xylia.

Quotes

 * "Be safe; Terraria needs you!"
 * "You must cleanse the world of this corruption."(Corruption world only.)
 * "You must cleanse the world of this crimson."(Crimson world only.)
 * "The sands of time are flowing. And well, you are not aging very gracefully"
 * "What's this about me having more 'bark' than bite?"
 * "So two goblins walk into a bar, and one says to the other, 'Want to get a Goblet of beer?!'"
 * "Why does [Name of Merchant] keep trying to sell me an angel statues? Everyone knows that they don't do anything." (Only if Merchant is present)
 * "I wish [Name of Arms Dealer] would stop flirting with me. Doesn't he realize I'm 500 years old?" (Only if Arms Dealer is present)
 * "Have you tried using purification powder on the ebonstone of The Corruption?" (Only if the Eater of Worlds has NOT been defeated - Corruption world only.)
 * "Have you tried using purification powder on the Crimstone of the Crimson"? (Only if the Brain of Cthulhu has NOT been defeated - Crimson world only.)
 * "Have you seen the old man walking around the dungeon? He doesn't look well at all..." (Only if Skeletron has NOT been defeated)
 * "I feel like [Name of Truffle] is a bit misunderstood, he really is a fun guy." (Only if Truffle NPC is present)

Only during a Blood Moon:
 * "I sell what I want! If you don't like it, too bad."
 * "Dude, is it just me or are there a million zombies out tonight?"
 * "I don't want you to buy my stuff. I want you to want to buy my stuff, ok?"
 * "Why do you have to be so confrontational during a time like this?"

Once the world is completely pure:
 * "[Name of World] is completely pure. You have done an amazing job!"