The Operator is one of the nine NPCs introduced in the AlchemistNPC mod. She can be named Angela or Carmen.
She can move into an empty house with the Wing of The World placed in it, after the Eater of Worlds / Brain of Cthulhu have been defeated. If playing AlchemistNPC Lite, the Wing of The World is not needed. If you play with multiple other mods you might need to have a house with the Wing of the World in addition to a table, chair and lightsource.
She sells boss drops and some special EGO equipment. While in Expert mode, Vanilla and Modded Treasure Bags are also present in her shop. She will help you open the Xenomorphic Mystery Meteorite when you get it at the end of the first Blood Moon in your world.
She drops the APMC, also known as the AI Programming And Memory Chip, which can be used to spawn her back in if killed. The tooltip of the APMC will reference the name of the previous Operator, and when used it will spawn her back in with the same name.