She's from "The Warlord," as the daughter of the mostly titular character. "The Warlord" ran for almost a decade back in the 1970/80s by Mike Grell and others, and was hugely popular, but barely had any reprints.
The setting was Skartaris, which was originally a world in the inside of the Earth's crust, but later became another mystical realm connected to Earth and Atlantis. Power Girl visited it when she was trying to learn about her Atlantean grandfather (back editorial decided that she wasn't a kryptonian). Power Girl actually became "as close as sisters" with Jennifer.
The Warlord starts with a U.S. Air Force pilot flying into a hole in the Arctic. Long story short, he kills a bunch of people (and dinosaurs) with his trusty .44 and became a leader in the setting.
Characters from it were mostly contained to it when it was running. Jennifer has made a few appearances in some magic related books, but you can count them on less than two hands.
She recently appeared again in Trinity like >>100852051
alludes too, but once again in that hollow-earth fantasy setting.
If you ever watched that one episode of Justice League Unlimited about the Earth's core, that took place in Skartaris.