Who is the last person that A was in in Every Day?

On the last day of A's journey in the novel Every Day by David Levithan, A wakes up as Katie. We don't know much about this day, day 6,034, but once A wakes up as Katie, they decide to run away to disappear.

I wake up two hours away, in the body of a girl named Katie. Katie doesn’t know it, but today she’s going far away from here. It will be a total disruption to her routine, a complete twist in the way her life is supposed to go. But she has the luxury of time to smooth it out. Over the course of her life, this day will be a slight, barely noticeable aberration. But for me, it is the change of the tide. For me, it is the start of a present that has both a past and a future. For the first time in my life, I run (105).

Throughout the novel, A inhabits many people and begins to wonder what it would be like to be able to stay. Each day they wake up in a new body and live their life for a day without trying to change too many things. Especially after spending time getting to know Rhiannon, they want the chance to find a life of their own.
A tells Rhiannon that Reverend Poole has given them the answer as to how to stay put, but they must be willing to leave and not return. A sets Rhiannon up with Alexander to make sure she will have a real chance at love. When they wake up as Katie, they knows everything is set and run to live a new life.