He appears in some of Asimov's robot stories, most notably in the novels The Caves of Steel and The Naked Sun[?].
Warning: Wikipedia contains spoilers
Daneel is a robot constructed to be indistinguishable from a human being (see android). This enables him to help earth-policeman Elijah Bailey[?] to solve crimes.
In his last books Asimov tied the foundation and robot stories together to form a grand future history. In the last foundation book Foundation and Earth we find out thet Daneel has survived for some 20 000 years and has manipulated the galaxy with the help of his many robot allies.
He set up both the Galactic Empire and Gaia in order to create a society that didn't not need robots. He also guided Hari Seldon so that he would develop psychohistory.
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