Who is Count Hasimir Fenring from Dune and what is their importance?

Count Hasimir Fenring is a genetic eunuch and the Emperor's boyhood friend, as well as his current advisor. He lives on Arrakis with his wife Margot. Fenring is described as having a weasel's face with oversized dark eyes. He looks to be short and weak, and he punctuates his words with extended humming noises. At heart, however, he is a dangerous killer.