Warning: Wikipedia contains spoilers
We first meet Eto Demerzel in Prelude to Foundation, where he is the shrewed first minister to galactic Emperor Cleon I. He attempts to win Hari Seldon over to the Emperor's side. In the end, we find out he is really R. Daneel Olivaw.
He would appear again briefly in the epilogue to Forward the Foundation, which says he was one of the few in attendance of Hari Seldon's funeral. At the time, he was wanted for questioning by the Commission of Public Safety