Professor Harrison Wells:

On Earth-2, Harrison Wells is the pioneer of modern scientific advancements, creator of particle accelerator; shares differences with the Flash of Earth-2 (Jay Garrick) who gained super-speed after particle accelerator explosion took place.  Prof. Wells came to Earth-1 post singularity to help Flash defeat Zoom and thwart his evil designs.  

Dr. Light:

Zoom sent another meta-human assassin from Earth-2 to eliminate Flash, and it turned out that she is Barry’s ex-girlfriend Linda Park. Dr. Light is not entirely evil, but she has convinced herself that if she kills her look-alike, somehow she will be safe from Zoom’s tyranny.  

Zoom:

The speculation is rife that the Zoom at Earth-2 is noboby but Barry Allen himself. As he knows so much about Barry’s personal life that he sent his ex-girlfriend to kill him, it is perfectly possible that he is Barry Allen’s version of Earth-2 which sounds insane but makes for great television.  

Killer Frost:

It became clear from the last season when Flash broke through the time-space continuum and saw Killer Frost who is none other than Earth-2 version of Caitlin Snow.