Jack and Diane are advisers to Papa Khan. The pair run a drug trafficking ring in the Mojave and supply the Great Khans with drugs as well. Jack learned chemistry from the Followers of the Apocalypse when they visited the Khans and uses this knowledge to make the drugs, while Diane handles the business end of the operation. 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 and make her a prostitute.
Jack is noticeably more laid back than the other Great Khan members, putting much less emphasis on the tribe's warrior culture and often speaking in "stoner" slang. He even expresses sympathy for Jerry the Punk, despite Jerry thinking little of Jack, claiming Jack only passed his beatdown because he was "baked to the gills" at the time.
After Aba Daba Honeymoon is completed, Jack will start to stock his supplies with the newly-taught chems, saving the player character from having to gather or make them. However, if they complete the quest without teaching Jack the recipes, they will be unable to get the chems afterwards, as he will not stock them.
pcps3xbox360 When you give Jack a new recipe for a Chem, the screen will black out, and roughly a few hours pass. After that, the screen will zoom in (maybe to begin dialogue with him), and then zoom back out and have another time lapse. [verified]
pcps3xbox360 Jack's Speech challenge for the Oh My Papa quest (Speech check 60) can be repeated an infinite number of times, even after the quest has been completed. This challenge earns 60 experience points each time it is performed, allowing the player to gain infinite experience points. However, if you rescue the runner Diane said went missing, you will not be able to use this exploit. (Still works after patch 1.02 on xbox360) [verified]
xbox360ps3 After teaching Jack all of the chems in the quest Aba Daba Honeymoon and finishing Diane's portion of the quest, you may not be able to finish the mission or learn the Khan Trick unarmed attack. [verified]