3. He's Detective Eddie Thawne
This is a theory which doesn't make any sense whatsoever, but it remains one of the most popular. When it was revealed that a Detective Eddie Thawne would have a role in The Flash, fans of course assumed that he must be the Reverse-Flash hiding in plain sight. After all, he could have taken on this persona just to keep an eye on Barry and to torment him by dating Iris West. As time has gone on though, it's become more and more unlikely. However, the fact that the Reverse-Flash left only Eddie alive after escaping from the trap in S.T.A.R. Labs could be a clue to a bigger mystery. Viewers somehow saw the villain beating up Harrison Wells even though he's since been revealed as Reverse-Flash, so you already know that he can somehow make it appear as if he's in two places at once. Thus, the theories about Eddie being an ancestor of Eobard Thawne are obviously much more closer to the truth (killing him would have resulted in Reverse-Flash being wiped away). There's also the popular suggestion that Eddie is Cobalt Blue, the secret twin brother of Barry Allen who was stolen as an infant, but perhaps not an identical twin here. Nora Allen and Charlene Thawne gave birth on the same night, but when the drunken doctor botched the latter's delivery, he told the Allens that one of their children was stillborn and switched that child with the dead Thawne in order to avoid the wealthy family's wrath. This would certainly be an interesting dynamic to explore for Barry and Eddie.