Mechanic

She starts the game tied up in the Dungeon, and freeing her is just a matter of walking up and interacting. The catch is depth: she only generates at cavern level and below, and anything that goes that deep before Skeletron is dead gets torn apart by Dungeon Guardians, whose presence also overrides normal Dungeon spawns entirely. However much of the upper corridors you sneaked through early, she is a post-Skeletron rescue.

Her position is random and the place is a maze, so plan a real sweep rather than a quick look. A Lifeform Analyzer takes the searching out of it completely. If something kills her while she is still tied up she appears again later, and if she dies after moving in she returns as long as a house is free.

Every wired thing in the game comes through her shop, and there is no second source for any of it. Traps, automatic doors, wired lighting, critter farms and the teleporter networks most late-game bases are built on all sit behind this one rescue. Do it on the same trip as Skeletron rather than filing it under later.

Most of her stock is bought once and never thought about again. The exceptions are Wire and Actuators, which you buy by the stack because every build eats them, and the Junction Box, which is cheap and is the only clean way to cross two circuits without them talking to each other. Buy far more of all three than you think you need.

The wrenches come in four colors, and you only need the extra three once a build carries more than one signal on the same run of wall. Triggers split into things a player uses, like a Lever or a Switch, and things the world uses, meaning the pressure plates, which differ by what is allowed to step on them. Timers are how anything runs by itself, and the very short ones only turn up in her stock later, so an older world's habits tend to be built around the slower set.

Two items are quiet quality of life and easy to skip. The Mechanical Lens shows which wires actually run where, which turns debugging a circuit from guesswork into reading, and the Mechanical Ruler keeps a layout aligned. The Teleporter is the expensive line and the reason to come back with money rather than something to buy on your first visit, but it changes how a base is laid out more than anything else she sells.

Three of her lines only exist under conditions, and they are easy to miss because the shop looks complete without them. Talk to her while standing in a graveyard and she stocks Spectre Goggles, which appear nowhere else in her inventory. The Mechanic's Rod needs the Angler living in town and only shows on certain moon phases, so if it is missing, come back on a different night. And housing her in the right biome next to another townsfolk gets you that biome's pylon, which is usually the cheapest way into a teleport network long before real teleporters are affordable.

Living underground costs her happiness, but the ice biome takes priority when the game works out which biome a house belongs to. Put her somewhere snowy below the surface and it reads as snow rather than underground, so you get the depth without the penalty. She and the Goblin Tinkerer enjoy each other's company, and since both sell things you will buy over and over, keeping that pair happy pays for itself.

Unlike most townsfolk she is not helpless. She hurls a wrench at anything that comes near and catches it on the return, and her damage scales up as the world progresses rather than staying where it started. She will not carry you through an invasion, but she is not free food on a blood moon either. If she does die, there is a small chance she leaves the Combat Wrench behind as a weapon you can actually use.

• The reason she was tied up at all is the Clothier. He put her there while Skeletron had hold of him, and neither of them remembers it clearly, which is why they dislike each other without being able to say why. Her distaste for the caverns comes from the same incident.
• The cult behind the endgame forced her to build the mechanical bosses as replacement organs for Cthulhu. She never finished the brain, which is why the Brain of Cthulhu has no mechanized counterpart.
• Shimmer turns her into a goblin, which is purely visual and entirely a joke at the Tinkerer's expense.
• The wrench she throws and catches nods at Ratchet and Clank, and her line about checking whether the device was plugged in is the oldest joke in technical support.