Who is Marcus Claire Luyseyal from God Emperor of Dune and what is their importance?

Marcus Claire Luyseyal is a young, accomplished Bene Gesserit Truthsayer, Luyseyal dominates older fellow Truthsayer, Tertius Eileen Anteac, during their embassy to Arrakis during the Decennial Festival. Believing raw spice-essence to be a specific poison for sandworms (and wrongly assuming Leto has reached that stage), Luyseyal creeps up on him by millimeters. Leto sniffs out the evidence, confiscates the goods, and is amused as the women plead curiosity. Luyseyal loses all semblance of Bene Gesserit control and Anteac skillfully uses Voice to control her. Leto remarks that Luyseyal ought not to be allowed out in public without controllers.