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 a Boss (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.

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."
 * "That merchant keeps trying to sell me an Angel Statue. Everyone knows that they don't do anything." (Only if Merchant is present)
 * "What's this about me having more 'bark' than bite?"
 * "I wish that gun seller would stop talking to me. Doesn't he realize I'm 500 years old?" (Only if Arms Dealer is present)
 * "So two goblins walk into a bar, and one says to the other, 'Want to get a Gobblet of beer?!'"
 * "Have you seen the old man walking around the dungeon? He doesn't look well at all..." (Only if Skeletron has NOT been defeated)
 * "It is an evil moon tonight. Be careful." (Only during a Blood Moon)