Warning! MAJOR SPOILERS for The Flash season 4 finale ahead!
On tonight's season 4 finale of The Flash, the identity of the mystery girl first seen at Barry and Iris' wedding is finally revealed - she's Barry and Iris' daughter from the future!
After that initial appearance, Jessica Parker Kennedy's mystery girl continued to pop up throughout the season and always within earshot of someone from Team Flash. In one of those appearances, she was seen scribbling in a notebook in that strange Speed Force alphabet, and in another it was revealed that she was, in fact, a speedster. Both revelations and the odd manner in which she always behaved while around Barry and Iris only fueled fan theories that she must be a relation of Barry's from the future, either a daughter, niece, or granddaughter. And as it happens, those fan theories were exactly right, making any and all attempt at misdirection on the part of the show seem meager at best.
Having been correctly guessed months earlier, the grand reveal that the mysterious speedster - who in this episode helps Barry destroy the satellite DeVoe sends crashing to Earth - is actually Barry and Iris' daughter from the future, Nora Allen, does fall a little flat. Still, introducing a speedster daughter from the future isn't without story potential, and as is also revealed in tonight's finale, the reason why Nora has traveled to the past is certain to play an important part in next season's arc.
Previously, while theorizing that the mystery girl would be a relation of Barry's, different characters from the comics were put forward as possibilities; like Dawn Allen, Barry and Iris' daughter, or Jenni, their granddaughter. Both Dawn and Jenni go on to become speedsters, so while The Flash isn't strictly adapting either character, it's certainly possible they will still serve as an inspiration. Dawn and twin bother, Don initially hid their superpowers before becoming heroes themselves, the Tornado Twins; while Jenni went on to serve as a member of the Legion of Superheroes. It's possible, then, that Nora also acts a superhero in her own time, just like her father, and possibly after a period in where she has to hide her abilities, or she might be a member of some future team organization like the Legion.
At this point, there's simply too much we don't and won't know about Nora Allen until The Flash returns in the fall, but she did leave us with one clue. After revealing herself to her parents, Nora mentions why she's come - she made a huge mistake and now she needs their help. No one can argue she isn't Barry's daughter. But whatever mistake she caused it's likely something so big that it's endangered or even killed her parents, the rest of Team Flash, and whatever other heroes are around in the future. Otherwise, why would she need to travel to the past looking for help?
The Flash returns for season 5 this fall on The CW.