The Season 4 finale of The Flash packed a bunch of potentially exciting mysteries right into the tail end of the episode, although arguably its biggest came right in the middle. In the midst of Team Flash attempting to project Barry into The Thinker's mind via the in-labor Cecile, the psychic meta snapped and purposefully announced: "I found Thomas; he's been this way all the time." We've been wracking our brains ever since the moment trying to figure out who was controlling Cecile, who Thomas is, and what that all may mean. Here's the best we've got so far:
Thomas Is Caitlin Snow's Father
Thomas is Caitlin Snow's believed-to-be-deceased father, although that message seems to say he's not as dead as we all assumed. Caitlin's dad, who was never given a name, is believed to have passed after losing a battle against multiple sclerosis when Caitlin was younger. Snow seemed to react with shock to the news delivered by Cecile's weird voice, which leads us to believe that she recognized the name Thomas and was startled by the fact that he was found. The question is, who would care about this enough to try and alert Caitlin Snow?
Remember how Cecile had the ability to take on personalities and read others thoughts late in her pregnancy? Good. Now, remember that Killer Frost is somewhere deep within Caitlin but unable to come out and communicate with her other self? Finally, remember that whole bit where Caitlin remembered a repressed memory where she was Killer Frost as a child? The Flash just opened up a can of worms with that moment that could mean there are gaps of Caitlin's childhood she almost certainly doesn't remember. This would explain how her father may still be alive without Caitlin knowing it, although we have no clue how Killer Frost managed to find him while trapped away inside her body.
Thomas Is Caitlin Snow's Brother
Caitlin Snow is still contacted by Killer Frost in this theory, although her message about Thomas is not referring to daddy dearest. Instead, Killer Frost is alerting her other self to the fact that she found their brother, and also that he's been "this way all the time." We'll tackle that latter part in a bit, but first, why would anyone who watches The Flash think Caitlin Snow has a brother?
Remember Earth-2 Killer Frost? She had a brother named Charlie who died and began the cycle of events that led to that Caitlin becoming Killer Frost. Seeing as Earth-2 Caitlin had a brother, is it so crazy to think that Earth-1 Caitlin has one as well? Sort of, considering The Flash has never mentioned or alluded to Snow having a mystery brother, although other shows have managed to pull off similar plot points. Should Caitlin have a brother named Thomas, he may be a meta like her, which might explain why he's always been a certain way. That would explain the second sentence of the quote, but once again, we have no clue how Frost would've found anyone in her current state.