It's Eobard Thawne, from the 25th Century.
Thawne comes from a future where the Allens and the Thawnes are feuding families, right? This is where it starts. Barry's gonna end up with Iris, and Eddie is gonna despise him for it. Their breakup won't be amicable, and his friendship with Barry will crumble. He'll forever hate Barry for ruining his chance at happiness, a hate which will only grow when he discovers that Barry is the Flash. Based on last night's episode, he's not too fond of the Scarlet Speedster.
This feud between Barry and Eddie will carry on into the future through their families, eventually reaching Eobard.