Dryad

Info
The Dryad is a Town NPC that sells all thigs nature. She also gives off a Buff called the Dryad's Blessing, which increases your defense, and a debuff called the Dryad's Bane which negatively affects all enemies, including Bosses. It is extremely helpful to build the Dryad's house in your fighting arena, as she will give you an advantage.

To move in, the Dryad requires one boss to have been defeated. Unfortunately, this does not count King Slime,Queen Bee or The Wall Of Flesh. This means the easiest possible way to fulfil the Dryad's requirements is to defeat the Eye Of Cthulhu. This does mean you may have to fight The Eye without the help of the Dryad's Blessing.

The Dryad will also give you the percent of Crimson/Corruption/Hallow that plagues your world, and congratulate you when it is 0%. She is also useful when building a Surface Mushroom Biome, as when she is in a house there, she will sell mushroom biome seeds. Her potted plants look pretty cool, also.