Who is R. Daneel Olivaw from The Caves of Steel and what is their importance?

Daneel is a humanoid Spacer robot, highly advanced in comparison to the robots of Earth. Designed by Dr. Sarton, in his exact human likeness, Daneel's similarity to an Earthman allows him to inhabit the City without being recognized as a Spacer robot. His anatomical makeup is complete from head to toe. He possesses a makeshift stomach referred to as a "food sac" where swallowed food sits until he opens his chest and empties it into a receptacle. He is capable of smiling and breathing at a consistent rhythm, yet does not require sleep or food.