SPOILERS ahead for anybody who hasn't yet seen the midseason finale of The CW's The Flash.
Now that that's out of the way: Harrison Wells. What about that guy, huh?
Apparently he's the Reverse-Flash, in spite of having been in the room with the Reverse-Flash.
Or maybe he's just working with the Reverse-Flash? In any event, he's got the costume stowed away in his secret room, and attached the tachyon particle device to its chest to...energize it somehow.
Well, of course, the obvious answer is time-travel. It's been clear since the premiere that Wells was capable of time travel in some capacity, or at least had access to technology and information from the future.
Of course, that doesn't particularly track with him being Hunter Zolomon, the Reverse-Flash everyone suspected that he might be.
We'll backtrack a bit for those just joining the party.
There's more than one character who's called himself the Reverse-Flash or Professor Zoom. The first, Eobard Thawne, is a time-traveling supervillain who drew his powers from the speed force like Barry Allen did, who had comparable speed and powers but who used them for evil.
The second, Hunter Zolomon, was somebody who fancied himself a friend and aide to The Flash.
"My name is Hunter Zolomon. Despite what the public believes, I am the fastest man alive. I am Zoom. But I am not a Rogue. Far from it. I have taken the name and colors from Eobard Thawne -- the time traveler know as Professor Zoom -- in order to terrorize my friend. Wally West. The Flash. I suffered through tragedy. I lost my family to it. I understand the depths it will drag one down to. And only by surviving it does one become stronger. I will recreate myself to help my friend and in turn -- the world. I will do anything to make the Flash a better hero."
He wasn't technically faster than The Flash, though; his powers worked a little differently and involved manipulation of time, allowing him to basically move between seconds, appearing fast but in fact moving at normal speed, just in a way that made him nearly impossible to hit.
His backstory: He was severely injured following an attack by Gorilla Grodd and left paralyzed. He wanted then-Flash Wally West to travel back in time (The Flash can do that, especially using the Cosmic Treadmill) to stop the injury from happening, but Wally refused, not wanting to mess with the timestream. When Zolomon tried to use the treadmill himself to do it, the resulting explosion is what gave him his powers. He determined that in order to be a better hero and understand personal loss, Wally had to experience some of his own, and targeted Barry's loved ones, even causing a miscarriage in his wife.
It wasn't Zolomon who killed Barry's mom, though; that was Thawne.
In the Geoff Johns-penned Flash Rebirth, Thawne learned that Barry Allen had died, sacrificing himself to save the universe during the Crisis on Infinite Earths. Enraged that his enemy had died with dignity, Professor Zoom sent a subliminal message to Barry, who had briefly emerged from the Speed Force, he engineered Barry's return during Final Crisis.
Thawne then broke into the Central City crime lab, murdered two police scientists, and recreated the accident which originally had granted Barry his speed, creating the Negative Speed Force. Zoom then traveled through time on a personal vendetta against Barry Allen: He pushed a young Barry down the stairs, burned down his childhood home, left the door open so his dog was hit by a car, murdered his mother and attempted to stop his son's marriage in the 30th Century (thanks to the DC Wiki for that cheerful refresher).
Of course, Harrison Wells appears to be actually attempting to ensure that Barry dies in the Crisis, becoming very alarmed when time was briefly altered to make it so that he did not. He also didn't cease to exist when time was altered, suggesting that he may not be from the future.
So...is he Zolomon?
It's hard to say. Not only did the Reverse-Flash murder Barry's mother, but the implication that another speedster was there at the time -- Barry, or someone wearing his colors or accessing his portion of the speed field -- suggests that elements of the Flashpoint storyline, in which Barry successfully traveled back in time to prevent Thawne from murdering Nora, only to discover the damage done to time would be so massive he had no choice but to let her die, are in play.
Could the Reverse-Flash we saw tonight know that already? he did tell Barry that it was his mother's destiny to die that night. If Flashpoint already happened, it would be a clever way of avoiding the need to do an alternate-reality story (which can go hinky fast -- just ask Heroes) while still enjoying the fruits of the story in terms of character development.
You'll also notice that he never actually confesses to having killed Nora. He doesn't deny it, and he certainly uses it against Barry...but he never admits he did it.
"If you want to find that out, you'll have to catch me," he tells Barry when confronted about why he killed Nora. When Joe West asks him, he ignores it completely and then the only other reference to Nora's fate was the aforementioned "she was destined to die that night."
It's not impossible that both Reverse-Flashes are in play here. Could Zolomon, for instance, be actively aiding Thawne while pursuing his own agenda? That would explain how Wells could have been in more than one place at once without time-travel, which is the obvious answer at the moment. Of course, most people have been speculating that if it was Zolomon, it would be Wells while if it wasn't, it would be Eddie Thawne (Eobard is a 25th Century bastardization of Edward). Since he, too, was in the room with Zoom, either everybody is wrong or there's some time-travel going on there regardless.
...And let's not get into the fact that the jagged black at the bottom of the Reverse-Flash costume is reminiscent of Daniel West, the brother of Iris and current Reverse-Flash in the New 52.
Meanwhile, what about the thing with the costume? It seems likely he was infusing the costume itself with speed force (or negative speed force) energy; during the Wally West era, his costume eventually was made of speed force, meaning that he could draw from it for an extra boost and that he could more or less summon it at will without needing to change.