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 you a special unarmed perk called Khan Trick with which the Courier can throw sand in the eyes of your opponents, often effectively distracting them for several seconds.
If you can pass a 50 Barter check, you 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 you will get 50 XP and positive Karma.