Mechanic

The Mechanic is an NPC, which must be saved from the Dungeon. She will randomly spawn near you while you are exploring Dungeon. Talking to her in this state will free her, and she will wander the area until she gets killed by any nearby enemy or hazard, or until you move far enough from her. After that, she will move into an available house like any normal NPC.

Once the Mechanic has moved into a house, she will sell the following:

Quotes
During Blood Moon if Nurse is present: In hardmode: When found in Dungeon:
 * "Always buy more wire than you need!"
 * "Did you make sure your device was plugged in?"
 * "Oh, you know what this house needs? More blinking lights."
 * "Hey, (Player Name), did you just come from (Name of Goblin Tinkerer)'s? did he say anything about me by chance?" (Only if Goblin Tinkerer is present)
 * "Hold on, i've almost got WiFi working down here."
 * "Hey, watch where you're going! I was over there a little while ago!"
 * "(Name of Arms Dealer) keeps talking about pressing my pressure plate. I told him it was for stepping on." (Only if Arms Dealer is present)
 * "Oh, let me guess. Forgot to buy enough wire. Idiot."
 * "Just-could you just...Please? Ok? Ok. Ugh."
 * "I don't appreciate the way you're looking at me. I am WORKING right now."
 * "All I want is for the switch to make the... What?!"
 * "DON'T MOVE. I DROPPED MY CONTACT." (Reference to Paper Mario: The Thousand Year Door)
 * "But I was almost done putting blinking lights up here!"
 * "Thanks! It was only a matter of time before I ended up like the rest of these Skeletons down here."