The Watcher is an unnamed Vesper agent responsible for monitoring world events. At the request of Vesper One, she monitors several members of the Cahill family (Ted Starling, Jonah Wizard, Hamilton Holt, and Ian Kabra) to find a suitable candidate to act as a mole within the Cahills. She eventually settles on Ian as the best candidate.
It is unknown if she was involved in the scouting and recruitment of Sinead Starling, who eventually defected to the Vespers.