Who is Kate in the shack?

In The Shack, Kate is Mackenzie "Mack" Phillips' youngest daughter. She was brutally murdered in the woods near the shack by serial killer Willie through the use of a blunt object. Mack goes to the shack after receiving a note which he presumes is from God, and in the shack, he meets three personages of the Godhead as Papa (God the Father), Jesus, and Sarayu (the Holy Spirit), who show him their nature through interactions with him and reflections on his life and family with a focus on forgiveness.