Margo & Sergei

Margo wasn't playing hard to get. She simply had no time or interest in romance or, ahem, human contact. As far as we knew, she slept in her office.

I will say that her character felt very poorly written. In keeping with her character's ideals, she should have let the KGB do their worst to Sergei. Obviously, her allegiance to NASA came first.

Nor do I accept her later explanation (in season 4) that she shared secrets with the Russians because competition was needed. Absurd. All the more so because NASA already had competition from Helios.