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

The grandmother of Leto II, Jessica is Leto I's concubine. She is a graduate of the Bene Gesserit School, and designated to bear a daughter who will become mother of the Kwisatz Haderach—the male Bene Gesserit who can be in many places at once. Instead, she falls in love with a Duke and bears him a son, Paul Muad'Dib, who becomes Emperor and inspirer of a great bloody Jihad that the God Emperor Leto II's "Golden Path" is intended to correct. Jessica's memory is sometimes bound up with her daughter Alia's underground cult, to her dismay inside Leto II. Leto likes to greet the Bene Gesserit using Jessica's voice, largely because this annoys them. In Goygoa Village, Idaho runs into the widow of his predecessor and is shocked to see she is a slightly aged and more motherly version of Jessica.