While many fans have speculated that she's related to Barry and Iris, executive producer Todd Helbing says that fans haven't yet figured out who she is. Helbing doubts that "anybody ever is going to guess exactly what it is".
"Some of them get pretty close, some of them they hit like one part of it but they don't get other parts", Helbing said in an interview with TVGuide, "so it's fun to read".
The Mystery Girl's identity on The Flash season 4 remains one of the show's greatest mysteries - we've see the character (played by Jessica Parker Kennedy) on a few occasions, but to date the only things that we know for sure is that she has some connection to Barry and Iris, she understands some of the same odd symbols we saw from Barry at the start of the season, and it seems like she's intentionally trying to meet certain characters at specific time. She appeared with nearly every member of Team Flash, except at different points of the show, over a span of nearly four months. When pressed if any of the (many) fan theories about Parker Kennedy's character were correct, Helbing was pretty blunt. Thus far, the running theory is that she's Barry and Iris' future daughter Dawn, or granddaughter Jenni Ognats.
It's possible Mystery Girl does not exist in the comic books or has a deeper connection that fans might need to dig through. Based on her interactions with Barry and Iris, many fans felt that she could be the couple's daughter from the future. There's even a slight possibility that the Mystery Girl could be an entirely new character or a gender-bent version of the second Kid Flash, Bart Allen. After all, that couple is the one who is now pregnant, not the speedy newlyweds. The expectation is that if the Mystery Girl is Dawn Allen she'll work with Barry and the team.
The Mystery Girl's most recent scene, where she purposely interrupted a momentous conversation between Caitlin and Harry about Killer Frost, had villainous overtones.
What do you want to see from this Mystery Girl on The Flash? It seems fans will have to wait a bit longer to find out who this mystery girl truly is. And then a supervillain comes along and zaps you and all of a sudden it's your wife who has the powers and not you.