Joo Won

Thrown straight into a dangerous mission with none of his memories intact, a man must escape death while trying to figure out who he is, how he ended up here, and who is the mysterious voice in his ear calling him "Carter"?

Two wannabe spies break up when one of them gets fired. Years later, both end up being spies working for different units, so they have to keep their covers up in real life. Gil Ro uses his real identity, hence why he was seemingly fired, to spy on his rich father. So, when Kim Seo Won appears as a new employee, he wonders if she really is just a doppelganger of the woman he loved. (Source: MyDramaList)