So episode 15 of the Flash was a bit of a shocker. The episode started out with the emotional fallout of "Jay Garrick's" death at the hands of Zoom that concluded episode 14. Team Flash tried to temporarily set their grief aside and deal with King Shark, but a lot of characters really struggled to do that. Caitlin Snow looked pretty hurt and was so disturbed, that Cisco feared she was going to turn into Killer Frost. Barry also struggled with "Jay's" death and even broke into tears over it. At the end of the episode, Barry tells team Flash that they are going to find a way back into Earth-2 to take down Zoom and avenge "Jay's" death. After that pep talk, we see Zoom carrying "Jay Garrick's" dead body into his lair. He drops the seemingly dead body and pulls off his mask revealing himself to be Hunter Zolomon (the man that "Jay Garrick" claimed was his Earth-1 doppelganger).
This is where things get a little trippy. It is very likely that the "Jay Garrick" character we've seen throughout the season wasn't actually Jay Garrick. The real Jay Garrick is likely the older war veteran with the mask that we saw behind the speed-proof glass. That is likely why he used the POW tapping code to communicate his name with Jessie and Barry. I am assuming that he is able to speak, but the glass made it impossible for the others to hear him. It is also very likely that the real Jay Garrick still has the speedforce in him, but is much slower than Barry; and therefore, couldn't phase through the glass.
The real "Jay Garrick" is probably known by another name: Hunter Zolomon. He was likely a scientist that wanted to become like the Flash that he heard stories about in Keystone City (which might have had meta-human problems before the particle accelerator went off, which is why people are unsure of whether or not they should blame Well's for this). Maybe the real Jay Garrick was in Central City, but never showed his face. But in order to become like the flash, E-2 Zolomon started creating these drugs to increase his speed. Eventually, Zoom showed up and made a deal with him. If he would help him get the Flash from E-1, than he would be able to become the E-2 Flash. He got to do that for a little while, but unfortunately, the side effects of the drugs that he used to get his speed caught up to him and made him submissive to Zoom. Zoom was his doppelganger, and therefore had the same cell tissue that he would need to cure himself. E-2 Zolomon pledged himself with Zoom, because he wanted to survive.
Zoom made him show up to Earth-1 and pretend to be Jay Garrick, so that E-2 meta-humans would recognize him and make his story credible. Dr. Wells knew him, but had tensions with him, in part because they were competing scientists. This is why he tried to blame Dr. Wells for the meta-humans; he hated him so much that he wanted to ruin him publicly. But being pledged to Zoom, Zolomon had to spy on Team Flash and report to Zoom. This is why "Jay Garrick" went missing a lot. It is also why he built the speed cannon in star labs. In addition, he had to help Barry get better. This is why Zoom was so prepared for Barry's new lightning move in episode 6. He knew that move was coming.
Unfortunately, E-2 Zolomon started developing an affection for Caitlin Snow. He fell in love with her. At times he wanted to tell her everything. In Episode 13, he said to her, "Caitlin I like how you look at me with warmth and pride but I am not worthy of those…I did a terrible thing Caitlin." He then went on to imply that he took V-6 before and lost his speed because of it. But there was more to it than that, he felt the guilt of betraying team Flash through his work with Zoom. At the end of episode 14, he got his speed back and started healing all of a sudden with Velocity 9. At last, he appeared to be free from Zoom's grasp. Things got better when Barry, Cisco, Wells, and Jessie returned home from Earth-2 and the breach started to close. "Jay Garrick" was so happy and had the biggest smile on his face. Zoom could no longer make him betray his friends. At that moment, Zoom vibrated his hand through the breach and appeared to kill him.
So even if this theory is accurate (which if there's anything I've learned about the Flash last season it's that it is almost impossible to predict where it's going), there are still many questions remaining. First, Zoom's costume and voice would make a perfect Black Flash. Why would the writers use this for Zoom, when they could've saved it for the Black Flash? (it would be cool if Zoom has a connection to the Black Flash and that leads us to Season 3's big bad) How does Zoom have that super creepy voice (I don't think you can say that he vibrates his voice that way)? Why is his lightning blue? Was he ever injured by Grodd like in the comics and for that matter, what is his origin story? Is he more than a speedster (in the comics he would manipulate time around him)? Is he from the future? Why does he need so much more speed?
I guess we'll have to wait a month to start getting answers.