Metallo

Metallo is one of the most famous and recognizable Superman villains, and could easily go through a bit of a revision in order to show up on Supergirl. John Corben is a journalist involved in a near-fatal accident, who's brought back to life by becoming a cyborg with a Kryptonite power source. One could easily see Corben joining Supergirl as a fellow journalist at CatCo, striking up a relationship with Kara, and, over the course of the series, becoming the villain we all know as Metallo. That, or he can simply enter the series fully formed, trading the Superman-dominated Metropolis for Supergirl's home turf, National City.