2. September
September has shown up frequently throughout Fringe, having been there right at the beginning when he saved Peter in the lake. He's an Observer, but there's something different about him. While the other Observers are devoid of all emotion (apparently due to their implants) and have only one agenda in mind, September has always tried to help when he could (even taking a bullet for Olivia). With the most recent episode revealing Donald (who had helped Walter conceive the plan for eradicating the Observers) to be September we now know that he returned to help once again and can help the team fill in the missing pieces of the plan.
The Questions: Why is September different to the others? Why is he helping the team? When it finally come to it, will he really be able to help destroy his own kind?