Jemma Benjamin, 18, shared her first smooch with Daniel Ross in the hallway of his apartment in South Wales in April 2009, The Daily Mail reported.
The pair had been friends for weeks and were planning to go out to eat that night, an inquest heard Thursday.
"It was not a sexual relationship. We saw each other a couple of times a week," Ross told police. "We were talking and ended up kissing by the front door."
A few moments later, she collapsed.
The cause of death was recorded as sudden adult death syndrome, The Sun said. The syndrome kills about 500 people in Britain each year.
Friends and family say Benjamin was a shy teen with a passion for sports, including hockey and swimming.
"There's so many things I could say about her -– she was a beautiful daughter and a lovely person," father Dale Benjamin said, according to The Daily Mail.
Benjamin had no history of cardiac problems, but was stressed about upcoming exams at school, her father said.
"I rang her mother to see if she had epilepsy," Ross said, according to the Daily Telegraph. "She fell in and out of consciousness."
He called an ambulance but paramedics were unable to save the woman.
Ross has since moved back to his home city of Birmingham, England. For Benjamin's family, the grief at this mysterious death goes on.
"I am not able to put into words how much I miss her," mother Charlotte Garwood said.