Jillian is one of the biological daughters of Leonardo Da'Vinci Dufae and Esme Shenske. Her surrogate parents, the Mayers, illegally acquired her and her sister Louise from the embryo bank they were stored after the in vitro fertilization that produced Tinker. Like Tinker, she is descended from the Stone Clan domana Unbounded Brilliance. Louise Mayer is her fraternal twin.
Her surviving biological family members are Alexander Graham "Tinker" Bell, Orville "Oilcan" Wright, Forge of Stone, Louise Mayer, four of her unborn siblings (nicknamed Nikola, Chuck Norris, Green Jawbreaker, and Red Jawbreaker), their biological mother Esme Shenske and maternal aunt Lain Shenske. She is the foster child of the dream crow Gracie Wong.
Jillian inherited the ability to see magic and notably did not inherit the ability to see the future unlike her sister Louise. She might just have a weaker ability to see the future that hasn't kicked in, but Louise noted that she always seemed to get caught while Louise usually managed to get out of trouble. Whether she inherited the ability like her sister to access the Stone Clan stones is unknown.
Jillian was raised with her fraternal twin by George and Mackenzie Mayer in New York City on Earth. Like her sister, she is shy due to bullying that happened when she was younger. Unlike her sister though, she interacts more with others as she has learned to get through her nervousness and shyness by acting, using a character as a mask of sorts. This has caused her to be considered the cuter and more outgoing of the two.