The Flash S02E15 aired last night, and let's be honest, EVERYONE'S jaw dropped after we finally found out that Zoom is actually Jay Garrick. But I was thinking about it alot today and it started to become less of a surprise. so let's dive on into this theory!
The theory is this, the Jay Garrick that came to Earth 1 is one of Zoom's henchman that's actually been gathering information about the Flash and everyone else at S.T.A.R. Labs before getting murdered last night.
Okay, don't click the red X at the top right corner of your screen, I can explain!
You see, the man in the iron mask that we see in Earth 2 is the Jay Garrick whose speed was stolen by Zoom. Zoom then hired a henchman and disguised him as Jay and sent him to Earth 1 giving him instructions on what to do, why do you think that real Jay was banging on the glass in worry after Barry told him that "Jay" is safe and okay on Earth 1.
Then disaster struck! Jay (henchman) fell in love with Caitlin and Zoom somehow found out about it. He then got murdered by Zoom.
So now Zoom, I don't have alot to talk about him since we found out who he was last night, so this one's gonna be short. His real name is of course Hunter Zolomon in the comics and, of course, show but how could he be in Earth 1 if he is just like Jay Garrick's Reverse Flash. Well, it's simple. Hunter was born on Earth 1! He then grew up, found out about the different breaches and Earth's, traveled to Earth 2 and became Zoom there! It's science!