Nobody
There are people in this world who are Nobody. No one sees them. No one notices them. They live their lives under the radar, forgotten as soon as you turn away.
That's why they make the perfect assassins.
The Institute finds these people when they're young and takes them away for training. But an untrained Nobody is a threat to their organization. And threats must be eliminated.
Claire has been invisible her whole life, missed by the Institute's monitoring. But now they've ID'ed her and have sent Nix to remove her. Yet the moment Nix lays eyes on her, he can't make the hit. It's as if Claire and Nix are the only people in the world for each other. And they are—because no one else can really see them.