Reiff is an agent of the D.E.O.
When Supergirl was fired from the D.E.O. for failing to reveal her secret identity, Colonel Lauren Haley decided to discover her identity in order to bend her to her wishes. For this, Haley interviewed most of the agents who had worked with Supergirl to get the most information about her. Alex Danvers asked those who know Supergirl's true identity to keep it to themselves, and they all agreed.
Agent Reiff was questioned and ultimately gave in, telling the truth to Haley, for which she apologized to Director Danvers. Shortly following, after the D.E.O. was attacked by the Morae, Haley sent forward Agent Reiff, only for her to be attacked by one of the Morae. Reiff, along with Alex and several fellow D.E.O. agents who knew Supergirl's true identity, had that fact erased from their mind by J'onn J'onzz with their consent.