Before Season 1
When Danny proposed, Sydney decided to tell him of her real life working for SD-6. As a result of this, Danny was killed by SD-6 agents for fear of him being a security leak for them. His death acted as a pivotal moment in Sydney's life, as it ultimately led to her finding out that she was in fact working for the very enemy she was fighting. His death became her motive for her becoming a double-agent for the CIA.
Before his death, Danny was a university student studying medicine specializing in pediatric cardiology, a choice he made after his brother died from a heart condition at a young age. After dating Sydney for some time, he proposed to her in a similar fashion to a person they had once seen in a bowling alley - in a public place, singing 'Build Me Up Buttercup' very loudly (Truth Be Told).
His death was investigated by Sydney's reporter friend Will Tippin; this investigation is what ultimately leads to Will learning the truth about SD-6 and who Sydney and her father really are.