999

999 is the conspiratorial contact of Flavia Stabo in her investigations into the mysterious deaths and the construction of Vault 79.

Background
999 mist likely worked at the Clarksburg post office where they programmed a deadman switch to activate automatically in the event Flavia Stabo didn't make it back from her investigation of the v79 site in time. They also had a side business that had some major issues, causing them to have to leave town.

Appearances
999 is mentioned only in Fallout 76, introduced in the Wastelanders update.