I have two, so here's #1, which is the Non-Sekrit-Cosmic-Player one.
I would be willing to guess fairly confident that she's Western US, if not Central Valley or desert Cali/Vegas area. Nobody else bitches about coffee during an apocalypse, and Bethany, who's from where she's from, immediately picks up on the lattes, so I joke, but Modesto/Fresno/Bakersfield is not an unfair guess. Possibly even Salinas or Stockton.
Inevitably, daddy was evil, but she never mentions her father, so I think he did not so much exist as ran out on them or died young so that it was Lilah and slightly crazy mom, who I imagine was functional if not pleasant to live with until Lilah was into law school. I think Mom was quite possibly religious.
Lilah wasn't popular in high school; however, she wasn't a Willow or a Xander. She either led the bad girls clique, or she was a school paper/track (maybe volleyball) team person. Solid, well-thought-of, but not Cordelia. Dated regularly; probably had her first girlcrushes on a fellow team member.
I think she did hang out with the bad girls/boys starting somewhat early on; found people a lot more amenable than whatever self-righteous brigade she hung out with beforehand. I can see her hanging around with the black-turtleneck squad, if not being one of them. Had a tendency to be, "the hot friend of [X]" until second or third year of college, when she hit her stride and just started vamping the hell out of everything and had a lot of fun doing it.
W&H probably came around as a result of her bad boy friends and the stellar work on law review. Still cannot believe she didn't read the contract in six different ways, so they offered her something worth eternity, and it might have been her mother out of her hair. Might have been a discreet legal record clean-up and a six-figure salary to start. Don't know. That part makes more sense in the other backstory I have going.