Jack and Diane are advisers to Papa Khan. The pair run a drug-trafficking ring and supply the Great Khans with drugs. Jack makes the drugs, and Diane handles the business. It is implied that they have an emotional attachment to each other, or at least Jack does since he becomes upset when told that the Legion would put Diane into slavery.
After completing the quest Aba Daba Honeymoon, Diane will teach Courier a special unarmed perk called Khan Trick with which the Courier can throw sand in the eyes of their opponents, often effectively distracting them for several seconds.
If the Courier can pass a 50 Barter check, they can convince Diane that it would make more sense to try to sell helpful chems to the surrounding settlements; with a success, she will agree and the player character will get 50 XP and positive Karma.