Warning! SPOILERS for The Flash season 4, episode 20 ahead!
Since she first appeared at Barry and Iris' wedding, fans have long speculated this mystery girl (played by Jessica Parker Kennedy) is Barry and Iris' daughter or granddaughter from the future; possibly even an established character from the comics like Dawn Allen or Jenny Ognats. She shares an interaction with Barry at the wedding in where she seems overly familiar with him and the occasion, and she's since shown up at Jitters, always in close proximity to someone from Team Flash. The last time she popped up she was shown writing in a journal in that strange Speed Force language - the same language Barry was writing in when he returned from the Speed Force in the season 4 premiere.
The running theory that she's a descendant of Barry's only gets stronger with this week's reveal that she's also speedster - leaving a streak of red, yellow, and blue behind her - but tonight's episode might also be hinting that she isn't Barry and Iris' daughter, but rather Joe and Cecile's!
The final scene of tonight's episode, 'Therefore She Is', has Barry and Iris throwing a baby shower for Joe and Cecile with the whole gang in attendance. The mystery girl knocks at the door, delivering a package for Joe and Cecile, which she hands to them saying it's a gift from "someone who realizes time is precious." She also mentions that it looks to her like Cecile will give birth in 21 days - which is a creepy thing to say to someone in addition to being the date of the season 4 finale - before seeming to vanish from the door. But the mystery girl hasn't vanished, she's just run down the hall and around the corner, where she waits a moment before she really vanishes - in a streak of red, yellow, and blue lightning because she's a speedster.
Besides introducing the huge (but not entirely unexpected) bombshell that this mystery girl is a speedster - she does know the Speed Force language, after all - the episode drops in another possibility for who her parents could be: Joe and Cecile. While the theory that she's the daughter/granddaughter of Barry and Iris still holds strong, Joe and Cecile are already pregnant and it would be just like The Flash to capitalize on the pregnancy by revealing who their kid grows up to be in the season finale. And since The Flash has never been a show about subtlety, the most obvious answer is most likely the correct one.
There's also the sadly ominous line about time being precious, which is very much a line said by a child meeting parents who didn't live long enough to see them grow up. And that would actually make her earlier, oddly endearing but awkward interactions with the other members of Team Flash make a lot of sense. If her parents were to die while she was still young, Team Flash - and potentially Barry and Iris, specifically - would be the ones to raise her. But for Joe's, Cecile's, and most importantly our sake, let's hope we're wrong about that.
The Flash season 4 returns next Tuesday with "Harry and the Harrisons" at 8pm/9c on The CW.