Who is Nayla from God Emperor of Dune and what is their importance?

Nayla is a Fish Speaker personally assigned by the God Emperor Leto II to spy on the rebel Siona Ibn Fuad al-Seyefa Atreides, Nayla is a heavily muscled woman with distinctive features: square jaw, wide mouth, compelling pale green eyes, close-cropped ivory hair, straight nose, and thin lips. She moves her jaw while talking like a primordial animal. Her strength is legendary among the guards. Leto considers her his most useful assistant and has implanted an Ixian transmitter in her skull in order to communicate directly.