According to TVLine's description of the character, he is "Barry's slightly older, slightly geekier contemporary, a guy who is as smart as he is intuitive. As such, this newcomer doesn't buy Barry's nice guy routine and sets out to learn what he's hiding -- all while concealing some secrets of his own."
At the end of Season 2, Barry changed everything when he went back in time and prevented Reverse Flash from killing his mother. This mirrors the events of Geoff Johns and Andy Kubert's "Flashpoint," which saw Barry do the same to disastrous effect; after Barry changed his own personal history in this comic event, he not only lost his powers but encountered a bleak world where Thomas Wayne became Batman and Superman was detained by the government his whole life, among other things. As such, it's possible the show could open in an alternate timeline, though it's unclear if and how long the show will stay there.