Warning: The following contains SPOILERS for The Flash season 6 finale.
Cisco Ramon, STAR Labs’ resident engineer, was curiously absent from The Flash season 6 finale, and that’s because he was on a separate mission. While this may ultimately have been to the episode’s benefit, given how many subplots it had to resolve, it was still odd not to see him on-hand for the battle with Mirror Mistress.
The Flash season 6 has been a tumultuous year for Cisco, even ignoring the events of Crisis on Infinite Earths, which saw him briefly regain his powers to aid in the battle to save reality. He was also called into action by his mentor, Breacher, to help find the murderer of his old girlfriend, Cynthia, only to find out the killer was an evil version of himself from another Earth. Cisco later took it upon himself to wander the Earth after the Crisis was over, chronicling how Earth-Prime was different from Earth-1 and which villains had somehow been restored to life or set free as reality rewrote itself.
One of the villains that popped up after Crisis was Mirror Mistress, who had abducted Iris, Kamilla, and Captain Singh, putting them in the Mirroverse. In order to rescue them, Cisco realized they would need to build a perpetual motion engine. Thankfully, he had all the components he needed to do it but one, and the missing piece could only be found in Atlantis, which he had visited several months earlier when travelling the new Earth. This left Cisco unavailable to aid in the battle with Mirror Mistress in the Flash season 6 finale, as he was still in Atlantis getting whatever exotic equipment he needed to breach the dimensional barrier separating him from the woman he loved.
Of course, since The Flash season 6, episode 19 wasn’t originally meant to be the season finale, it’s possible that Cisco would’ve returned and helped defeat Mirror Mistress before the season ended, thus having him take part in the original season 6 finale. But since The Flash season 6’s production was delayed due to the coronavirus pandemic, Cisco’s role was cut down in the end.
After all, in addition to finding a way to break into the Mirroverse, the biggest plot point for Cisco in the second half of The Flash season 6 has been finding a way to create an artificial Speed Force. This came about in the wake of the original Speed Force’s destruction. And it’s possible that whatever device Cisco recovers in Atlantis could be used to build a new Speed Force. Theoretically it was possible, as the Reverse-Flash Eobard Thawne had done it once before when creating his Negative Speed Force, but going about it using 21st Century technology proved a challenge that tested even Cisco’s ingenuity.
