The following is based on Van Buren and has not been confirmed by canon sources.
Jillian McKinley is one of the vital escaped Tibbets prisoners. Like many of her escapee comrades, she carries one of the New Plaguemutations – a piece in a large and lethal puzzle that the player needs, and the same virus that inhabits player’s own body.
Jillian is a striking woman in her late thirties, despite her unkempt, "wasteland" look when the player finds her. Her looks are only exceeded by her intelligence and skill with mechanics and science (it’s rumored that her family came from an actual vault, which explains her high mechanical and science knowledge, but she never talks about her family – possible PC dialogue quest). It is because of this intelligence, and her enviable skill with all things mechanical, that her life was spared the ghoul procreation process; a process that is usually fatal to humans. The ghouls found her ability with tools and cognitive thinking both refreshing and useful, almost to the point of making her an "honorary" ghoul. So, the ghouls charged her with gun smithing under the direct supervision of Hank, the ghoul gun and ammunition smith. Jillian made no protest since the alternative was probably a grotesque and horrifying death.
Jillian was abducted by a retrieval robot not long after the Rusty Hooks found Bloomfield. Like many of her fellow prisoners, she headed out in a direction she was somewhat familiar. Day and night she braved the wastelands, evading creatures of nightmarish size and grotesqueness. She thought she might actually make it to her home, Bloomfield Space Center. However, a few days before she reached her beloved home, Jillian was captured by a ghoul patrol and taken to The Reservation as a slave and potential procreation catalyst. Luckily for her, the ghouls discovered her high intelligence and skills in mechanics, and decided to make her a slave.
Acquire Jillian - The player will find Jillian sweating away in a radiation suit in the little trade area on the surface of The Reservation. It will be made clear that the player may not speak to any slaves. However, the player may inquire about purchasing Jillian from Hank, though it will take a very charismatic person with a high speech skill to even get Hank interested in selling Jillian (Hank is far too fond of Jillian, and even shows a bit of affection towards her, mostly because of her skill, which he admires and respects). Other options of acquiring Jillian is proving to the ghouls that the player can be trusted by freeing Harold from his capture and returning him to The Reservation, by foiling a trade scam that a human trader is conducting on the ghouls, by talking the guards into taking the player to meet Willem Clark by saying that the player knows about a hi-tech launch platform that is fairly functional, or by blowing the shit out of the ghouls, though the last option would be very tough due to the sheer number of ghouls and their formidable equipment.
As far as Sid is concerned with Jillian, she couldn’t return sooner. He will be ecstatic if the player brings Jillian back to Bloomfield so he can return to his life of background obscurity. 
Jillian McKinley did not appear nor was mentioned in any of the published games, but was to appear in the canceled Fallout 3 project by Black Isle Studios codenamed Van Buren. It is not certain if she will appear in any future Fallout game.