Who is the Mystery Girl?
This is arguably the biggest question of the Arrowverse this season, particularly on The Flash. Since her first appearance during the Supergirl hour of "Crisis on Earth-X" fans have been trying to figure out who the young woman with the nervous, goofy demeanor of Barry Allen and Iris West-esque looks actually is with most theories pegging her as either Barry's daughter or granddaughter from the future. Now, a new theory suggests that the reality might be far more sinister: is the mystery girl really Reverse Flash?
On the surface it sounds like the most ridiculous possible explanation, but when you take a moment to look at some of the details, this Reverse Flash theory starts to hold some water. First, a brief primer on Reverse Flash. The Reverse Flash's real name is Eobard Thawne and he's descendant of the late Eddie Thawne from the 22nd century. Obsessed with The Flash of the 21st Century (that's Barry) he went so far as to recreate the accident that created The Flash to give himself powers. This let him travel back into time where he discovered that it was his destiny to become Flash's nemesis leading him to, eventually, despise Barry. He travels back in time to kill Barry and erase him from history but kills Nora Allen instead. He ends up losing his connection to the Speed Force at some point, becomes trapped in the 21st century, kills the real Dr. Harrison Wells, and sets in action the creation of the Flash. Barry and Team Flash defeat Thawne and erase him from existence, but Thawne manages to come back and remains a foe of the hero.
So how does that translate to the Mystery Girl? Well, fans noticed that not only do we first see Mystery Girl during the "Crisis on Earth-X" crossover, but we also see Reverse Flash as well. Both characters appear to come out of nowhere and both disappear equally as fast. There's also the matter of something Thawne tells Barry in the final hour of the crossover. Thawne tells Barry "I wonder what face I'll be wearing next time we meet, Flash." While most fans took that as a jab at the fact that Thawne wore the face of Harrison Wells in his first encounters with Barry as well as during the crossover and we know he can take on different appearances, it's also possible that he was directly foreshadowing coming to Barry in a completely different form -- that of the Mystery Girl.
It's a curious theory and one that, while incredibly different from previous ones, is somewhat plausible. However, it's equally as possible that while the Mystery Girl isn't Reverse Flash herself she may well still be a Thawne. In comics, Barry and Iris have twins, Dawn and Don Allen. While Dawn goes on to marry Jeven Ognats and give birth to Jenni, who later becomes the Legionnaire XS, Don marries Meloni Thawne. They have a son, Bart Allen. It's not completely crazy to think that the Mystery Girl could be a take on Bart Allen, bringing Reverse Flash back into the equation in the process.
The Flash airs Tuesdays at 8/7c on The CW.