Who is the Seeker in the novel, This Is How You Lose the Time War?

The seeker is a manifestation and representation of fate. Whenever Red reads a letter from Blue that involves her manipulating the environment around her, the seeker will appear shortly after to put things back into their places. Plus, the seeker is the one who is hunting Red when the Agency realizes that she may perhaps be doing things that are not sanctioned and go outside the bounds of what they are trying to create.