She's from an alternate 2013 where time travel, teenage cops and all manner of hi-tech gadgetry like jet packs are the norm (I would kill to travel by jet pack!).
I'll get back to the story later because it wasn't the narrative, or even the concept I was particularly interested in when I pre-ordered my copy of Rocket Girl by backing Amy Reeder and Brandon Montclare's Kickstarter project. I was excited for the artwork. At the time I had just purchased Batwoman Vol. 1: Hydrology which as well as being one of my favourite incarnations of Batwoman, features some phenomenal artwork from Ms. Reeder.