Who is he? Multiple men have had his name–Eobard Thawne, Hunter Zolomon, Daniel West–but he’s exactly what his name suggests: the evil version of the Scarlet Speedster. Depending on the identity, he’s either a time traveler from the 25th century (Thawne), or is obsessed with making the current Flash better than he already is (Zolomon). More recently, he showed up in the first season of the CW’s The Flash under the identity of Harrison Wells (who turned out to be Thawne, because that’s not convoluted).
Why include him? The Flash’s villains didn’t really get their chance to shine in the original Injustice, so including them in the roster for Injustice 2 would be nice. Each version of Reverse has a different look to them, and if Netherrealm brings back the multiple fighting styles from Mortal Kombat X, the game could play off of that by having each version have their own special ability. Plus, c’mon, who wouldn’t want to see Flash fight his literal reverse?