Dryad

She needs an empty house and one dead boss, and it does not matter which of the early four it is: the eye, either flavor of world evil, or the skull in the dungeon. That makes her one of the first arrivals in almost every run, usually turning up while you are still in ore armor. If she dies later, the boss requirement no longer applies and she comes back on her own once a house is free.

Ask her for a status and she reports how much of the world has gone over to the evil biome and to the Hallow, counted from the actual blocks in the world rather than from anything you can see. There is no other in-game way to measure that. In late Hardmode it is the number that tells you whether an afternoon of cleanup actually moved anything, and whether it is worth continuing. If a world has both Corruption and Crimson she separates them and then gives you the combined figure. Drive all three to nothing and asking for the status is what awards the achievement for it.

A blood moon flips her stock. The purifying powder disappears and the evil powders and seeds take its place, so the night you would rather be indoors is the night to shop. In Hardmode a graveyard does the same job in reverse: stand among enough tombstones and she will sell you seeds for whichever evil your world did not generate, which is how you grow both sets of ores and drops without ever making a second world.

Two other lines are location-locked rather than time-locked. Mushroom seeds appear only while she is standing in a glowing mushroom biome, and ash grass seeds only in the underworld. For the mushroom trip a Queen Statue is far quicker than building her a second house, and those seeds are the shortcut to the surface patch the Truffle demands before he will move in.

Planter boxes are the standout and they no longer wait on boss progress, so a row of them beside your bench takes most of the hunting out of potion brewing: every herb grows where you can see it and harvests without a trip. The Dirt Rod is cheap and quietly one of the better building tools she carries. Acorns and Sunflowers are utility rather than decoration, since sunflowers push back monster spawns around a base. Hallowed Seeds are how you start a controlled hallow rather than letting it eat your world at random. The potted trees rotate with the lunar phase, so a collector filling out all of them is looking at a week of in-game nights rather than a single shopping trip.

Threaten her and she casts a ring of leaves that expands outward in stages, ending in a sweep far wider than any other town NPC can reach. Anything of yours inside it, players and other townsfolk alike, picks up a buff that adds defense, regeneration and a thorns effect, while enemies with line of sight take damage over time. It runs for a while if she is left alone, and cuts out immediately if she is hit, so she is worth housing behind cover rather than at the front of a defensive line.

The effect also scales with your progress. Each major boss you put down strengthens the damage side of it, which is why her ward feels negligible in the first hour and genuinely useful during a late-game blood moon. Higher difficulties raise the defense she hands to the other NPCs rather than lowering it, so a cluster of houses around hers survives events better than a spread-out town.

Carry a Joja Cola and her dialogue starts hinting at it. Hold it in your hand and the status option is replaced with an offer to purify it, which consumes the can and produces a small crossover scene ending in a Stardrop you can pick up. Separately, dunking her in shimmer changes her appearance permanently until you dunk her again, with no effect on her stock or dialogue.

Her lore is unusually heavy for a shopkeeper. She is written as the last of a race that fought Cthulhu, lost, and crippled it by tearing out the parts you spend the game fighting: the eyes, the spine, pieces of the brain. She also mentions being five hundred years old in one of her lines, and she does not appreciate the Arms Dealer.