Who is Eto Demerzel (a.k.a. R. Daneel Olivaw) from Forward the Foundation and what is their importance?

Eto Demerzel is the First Minister to Emperor Cleon I, Eto watches over Hari Seldon as the famed mathematician labors to turn his theory of psychohistory into a viable political tool. The relationship begins in Prelude to Foundation, where it is complex and mysterious, involving multiple personae. Only in the final chapters does Hari figure out that his friend and protector is a robot, R. Daneel Olivaw, so perfectly humaniformed that his nature has gone undetected for twenty millennia.