Robin Brooks (Michelle Nolden) is an Assistant U.S. Attorney who works primarily out of L.A. and often collaborates with Agent Don Eppes and his team. This is rather convenient, considering that, for most of the time she's seen onscreen, the two of them are in a relationship. In the season 6 (and series) finale she accepts Don's proposal and becomes his fiancée.
Robin comes with considerable baggage of her own, and she and Don prove that "the course of true love never did run smooth". There is equally no doubt, however, that she brings out parts of Don that would otherwise remain sequestered, such as his love of Sullivan's Travels.