Katya, Archangel of Balance, a being that Desmond both appreciated and feared. Although Desmond had learned about the inability of the Celestials to commit evil, this still did not allow him to lower his guard against this woman. The reason was very simple, being a completely different life form from human beings, Desmond's views on good and evil couldn't be the same as Katya's. Therefore it was difficult to predict if the archangel of balance was dangerous or not.
On the other hand, Desmond knew that Katya could be incredibly cunning in her way of doing things, so he didn't believe for a second that Katya was here by accident or coincidence; her calling Lilith by name only confirmed Desmond's suspicions.
