The Flash Season 2: 7 Theories On Zoom's True Identity

4. The Rival

It's Jay Garrick who arrives from Earth-2 to warn Barry Allen of Zoom, telling him that for some time now he has been fighting him in his own dimension. If you take that, along with the fact Zoom is a speedster, and then look at the comics, one character who would fit the bill is Edward Clariss, aka The Rival. He was a professor at the university attended by Garrick, and attempted to recreate the formula that gave him his speed. With his work rejected by the scientific community he became a villain, but was eventually defeated and then trapped inside the Speed Force. They could follow this angle, given he's coming from Earth-2 and Jay Garrick has been fighting him. They could also have the cause of his appearance now be a result of the singularity, opening up the Speed Force for him to emerge. His jealousy and anger would explain why he wants to eliminate speedsters, and with Barry and now Garrick in Earth-1 it's natural that he would follow.