The Reverse Flash is the Arrowverse version of the Flash's arch enemy Eobard Thawne aka Professor Zoom who is known for coming from the future to stir up trouble for Barry Allen and his reputation as the Flash
In The Flash 2014 series he first travels back in time to kill Barry but ends up killing his mother Nora out in anger over Barry escaping him and when he was unable to get back to his timeline, he kills Earth One Harrison Wells and his wife for his identity and spent 15 years working with Barry, Caitlyn and Cisco while secretly getting all the equipment he needs to travel back into the future. When all of that failed due to Barry's intervention he threatens to kill him and everyone in Central City that it caused Eddie to kill himself out of fear over the Flash's incapability to defeating his mother's killer thus erasing the villain from existance.
Now while the details on his return are yet to be revealed, most speculation has it that the Reverse Flash in the midseason 2 will be from the future of Earth 2 because it's impossible for his Earth One counterpart to return after being erased from existance in the season one finale but however the Reverse Flash will end up returning is most likely to depend on how Barry will succeed saving Patty from Zoom in the episode before "The Reverse Flash Returns";
That if Barry travels back in time to save Patty and reveal his identity as the Flash and everything that's been going on to her without loosing his powers to Zoom or getting beaten up and heavily bruised by him from before after remembering what Patty said to him about traveling back in time and cheating death on their first date and without letting anyone or anything persuade or interrupt him from doing so, it will tempt Zoom in the newly changed timeline into traveling back into Earth One's past and brings the E1 Reverse Flash back into existance which will change the timeline in Earth One
or If Barry successfully rescues Patty from being tied up in where Zoom is holding her hostage either by distracting him with his duplicate illusions and outrunning him to reach the place where Patty is held hostage and frees her before he finds a good place to hid her before reengaging Zoom or with Jay Garrick injecting himself with more Velocity 6 serum (aware of the consequences of how Patty's fate would affect Barry) or Wally West (having gotten his powers from the particle excelerator explosion while he was trying to meet up with his sister and father in Central City that day) will save her while Zoom is busy fighting Barry that when he learns that there is another Speedster just like the Flash in Earth 1 he will travel back to Earth 2 to summon the Reverse Flash from the future in that Earth to help him kill them all and rob them of their speed
Or with Barry now said to be dealing with a metahuman that is able to slow down time, Zoom will hopefully just kidnap Patty and hold her hostage along with Jesse in Earth 2 just to make Barry desperate on becoming stronger and into making it through the breaches to save her and however the Reverse Flash will return will be revealed.
to find out stay tuned for the midseason premier on January 19th or don't if you're still bothered by the promo to the 33rd episode of the show and maybe wait until February once the news on the outcome passes
