Who is Daniel Hecht?
 

Danny was a medical student whose marriage proposal to Sydney followed a two-year relationship. He was still unaware of her affiliations to SD-6 until she revealed the truth soon after their engagement. He found this news about their future together deeply upsetting.

When Danny left an inebriated message on Sydney's answering machine while she was on mission in Taipei, he made the mistake of mentioning that he knew she was a spy. A wiretapping SD-6 security agent promptly reported the matter to Sloane, who ordered Martin Shepard to "terminate" Danny.

Officially, it was Danny's assassination that alerted Sydney to the truth about her employer, and prompted her to seek revenge upon SD-6 with the help of the "real" CIA.