SPOILER WARNING: This article contains major spoilers for "We Are the Flash," the Season 4 finale of The Flash.
The Flash has finally revealed the identity of Season 4's Mystery Girl, who debuted in the four-show Arrowverse crossover "Crisis on Earth-X." As Team Flash celebrated the birth of Joe and Cecile's daughter Jenna Marie, she burst into their home and declared herself to be Nora West-Allen, the future daughter of Barry and Iris.
Just three episodes prior in "Therefore She Is," Nora revealed her speedster powers to the audience, indicating that she may be from the future. However, it wasn't until the Season 4 finale that she used them within sight of Barry and the others. As Barry raced to stop a satellite from crashing into Central City with a sonic punch, she joined him. This likely saved his life, as Marlize DeVoe pointed out the impact should have killed him.
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After Joe, Cecile and Jenna arrived home, Nora knocked on the door and invited herself inside. "This house is bitchin'," she said as soon as crossed the threshold, echoing one of the nonsense lines Barry wrote down just after he was released by the Speed Force in the season premiere. At that, everyone turned around to look -- and everyone recognized her, pointing out the times they bumped into her throughout the season.
When the shock wore off, Barry asked where Nora had gotten her jacket. She told him she had gotten it from Iris, who denied lending it to her. That's when she revealed she was from the future. She then confessed she had made a big mistake, though she didn't reveal exactly what that was. The fallout of her mistake will likely be explored in Season 5.
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"Barry certainly learned his lesson about time travel and the effects that it can have. She comes back for a specific reason, not only to see her parents and meet everybody on the team, which you saw all throughout the season — there’s four specific times that she came back, and you’re gonna learn about why she chose those four times and how they’re gonna play into not only the mistake that she made, but the consequences for somebody like her, a speedster traveling from the future to the past, and what that means for Barry and the team," executive producer Todd Helbing revealed to Entertainment Weekly.
Nora Allen does have a comic book counterpart, but she is a very recent character. Debuting in 2017's Justice League #26 by Bryan Hitch and Fernando Pasarin, Nora is the daughter of Barry Allen and Jessica Cruz, a Green Lantern. Like her father, Nora is a speedster on the Justice League, though she operates under the name Cruise. She has appeared in only seven issues to date.
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Joe and Cecile's daughter Jenna may also have a counterpart from Hitch and Pasarin's Justice League run. As it were, this future version of the Justice League includes a member named Jennifer Allen, who is the daughter of Barry and Jessica and is thus Nora's sister. In the comics, Jennifer Allen is a Green Lantern. At the time of writing, it is unclear if the two are related in any way, though their names are certainly similar.
Airing Tuesdays at 8 p.m. ET/PT on The CW, The Flash stars Grant Gustin as Barry Allen, Carlos Valdes as Cisco Ramon, Danielle Panabaker as Caitlin Snow, Candice Patton as Iris West, Jesse L. Martin as Joe West, and Tom Cavanagh as Harry Wells, with guest appearances from Keiynan Lonsdale as Wally West and Hartley Sawyer as the Elongated Man.