Dryad

Dryad is an NPC that will move in to an empty house once you've killed a boss. She only moves into the world in which the boss was killed. The easiest boss to kill would be the Eye of Cthulhu or, if you have a flail weapon, the Eater of Worlds.

This Dryad is also 500 years old, as she reaveals in conversation. She's one of two female NPCs, the other being the Nurse.

Some of the Dryad's items are very useful against the corruption. For example, purification powder destroys corruption, and sunflowers keep it from spreading. However, upon the night of a Blood Moon, her wares become "corrupted" and become Corruption-based counterparts to the normal stock. (Grass Seeds become Corrupt Seeds and Purification Powder becomes Vile Powder, at the same, if not, similar prices.*needs price confirmation*)

Quotes

 * "Whats this about having more 'bark' than bite?"
 * "So two goblins walk into a bar, and one says to the other, 'Want to get a Gobblet of beer?!'"
 * "You must cleanse this world of its corruption."
 * "The sands of time are flowing. And well, you are not aging very gracefully..."
 * "Be safe; Terraria needs you!"
 * "Have you seen the old man walking around the dungeon? He doesn't look well at all..." (Only when the Clothier is not present)
 * "It is a evil moon tonight. Be careful." (Only during Blood Moon)
 * "I wish that gun seller would stop talking to me, doesn't he realise I'm 500 years old?" (Only if Arms Dealer is present)
 * "That merchant keeps trying to sell me an angel statue. Everyone knows that they don't do anything." (Only if Merchant is present)