Spoilers ahead for Episode 20 of The Flash Season 4, "Therefore She Is."
The Flash has had a lot going on in Season 4 thanks to all things associated with The Thinker's plot for humanity with the Enlightenment, but the show has still found the time to tease the introduction of a character who could be a very big deal. The young woman currently known only as Mystery Girl made her debut back in the big Arrow-verse crossover as somebody who seemed a little bit too familiar with and invested in Barry's life, and she's popped up several times since. She made another appearance in "Therefore She Is," and it resulted in a major reveal about who she is and her potential importance. Mystery Girl is a speedster!
Here's how "Therefore She Is" dropped the big reveal. At the end of the episode, Team Flash was assembled at Barry and Iris' apartment to throw a surprise baby shower for Cecile. All was going as well as possible considering Cisco had just broken up with Gypsy, Caitlin was seemingly resigned to never getting Killer Frost back, and The Thinker (with Marlize) had just scored a major win. Then there was a knock at the door, and Joe opened it to find Mystery Girl (played by Jessica Parker Kennedy) dressed as Delivery Girl to drop off a diaper bag as a gift (and do a super bad job of covering up the fact that she knows more than she's letting on).
Mystery Delivery Girl took one look at Cecile and proclaimed that she looks like she's 21 days away from giving birth. She handed over the diaper bag to Joe and Cecile and said it was from somebody who wanted them to know how special they are. Then Iris walked toward her dad and Cecile. When Mystery Girl saw Iris, she beat a hasty retreat, and when I say hasty, I mean super hasty. By the time Iris looked out into the hallway, Mystery Girl had turned a corner and was safely out of sight. She then raced off with purple and gold lightning trailing behind her, revealing once and for all that she's a speedster.
If fans weren't suspicious that Mystery Girl was related to some of the Flash characters before "Therefore She Is," they should be now! The very fact that she's a speedster means that she could very well have a connection to one of the (admittedly many) speedsters that have popped up on the series before, and her clear investment in the members of Team Flash points toward her being related to either Barry or Wally as the speedster. Given that Kid Flash isn't even on the show anymore and The Flash is about The Flash, the odds are that Mystery Girl is related to Barry.
Mystery Girl's lightning may be the clue as to how exactly she's related to Barry, if that is the case. Her lightning seemed to be a combination of gold and purple. Barry's lightning is and has always been gold, and Iris' lightning was purple during her temporary stint as the speedster savior of Central City earlier in Season 4. The Flash seems to be hinting quite heavily that Mystery Girl is Barry and Iris' daughter. It would certainly explain why Mystery Girl fled as soon as she spotted Iris. There could be quite a lot of baggage there, even if Iris wouldn't recognize her as a speedster who has probably engaged in some time travel shenanigans.
If Mystery Girl is Barry and Iris' daughter (or granddaughter), that raises the question of how superspeed powers are passed along. Is Barry's superspeed enough for him to father a speedster? Or was Mystery Girl conceived while Iris had her speedster powers? Does Iris have some kind of lingering speedster energy that, when combined with Barry, produced a little bundle of speedster joy?