Okay, in our "Character" section, we explore the possibility of Seldon being the protagonist for all the stories in entire Foundation. But there's also a good case to be made for Seldon as the Guide/Mentor entity. Feel free to read both and come to your own decision on this one.
Seldon's Plan guides the protagonists of every story in Foundation by…not really guiding them at all. Let us explain. Seldon has already set future events in motion by mapping the time between the fall of the Galactic Empire and the coming of the Second Empire. All the characters need to do is allow things to happen as they will, further the development of science, and occasionally—during a Seldon Crisis—make the rational decisions.
This proves surprisingly hard for some characters, who try to manipulate events to their advantage. But those who use Seldon's Plan as a guide generally do much better.