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 or Skeletron.

Once the Dryad has moved into a house, 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."

She also sells the following:

Quotes

 * "Be safe; Terraria needs you!"
 * "You must cleanse the world of this corruption."
 * "The sands of time are flowing. And well, you are not aging very gracefully."(A reference to "Prince of Persia, Sands of Time")
 * "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 statue? Everyone knows that they don't do anything." (Only if Merchant is present) (There is a typo in the message)
 * "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 Eater of Worlds has NOT been defeated)
 * "Have you seen the old man walking around the dungeon? He doesn't look well at all..." (Only if Skeletron has NOT been defeated)

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?"

Trivia
One of the Dryad's names, Celestia, might be a reference to the alicorn in My Little Pony Friendship is Magic who is a sun goddess and the elder sister of alicorn sisters, (Princess Celestia & Princess Luna a.k.a. Nightmare Moon).