Karen, a Form Five student at Diocesan Girls' School, is the first Hong Kong athlete to win a medal at the Junior and Cadet World Fencing Championship, which this year were held in Croatia.
She went into the competition in fine form; in March, she won three gold medals at the Asian Junior and Cadet Fencing Championship, in Bangkok, Thailand. "I didn't get overconfident; I knew the level of competition was different.
"Hong Kong fencers who do well at the Asian event often get to the last 16 at the World event. That was my goal."
Yet Karen stunned the top seed, Hungary's Lili Drajko, in the quarter-final to guarantee Hong Kong's first medal in the event.
"After that win against Drajko, I was overjoyed," she says. "I celebrated in a way that might have led others to think I'd already won the title.
"I'd been so nervous. My coach told me not to think about fencing too much. So, when I wasn't competing, I stayed in my hotel room watching an old TVB drama, Triumph in the Skies, to take my mind off things. It really worked; I felt less tense."
A chilled-out Karen won the title after beating Theodora Goudoura, of Greece, 15-14, in an exhilarating, close final.
Karen, a keen 100 metres and 200 metres runner while at primary school, took up fencing at the age of 10. In Form One, she switched from using a foil to a sabre, which allows fast scoring not only with the point, but the edge of the blade, too.
"Since I had explosive power and fitness from my sprinting, coaches thought the sabre would suit me better," she says.
The change proved a success, but she waited until September 2011 to join Hong Kong's junior team because her father was diagnosed with lung cancer in 2009. "He was really ill, so I gave up all overseas competitions to could stay with him every day," Karen says.
Her father died in 2010, about a year after his diagnosis. She says: "I was very young and didn't cry too much when he passed away.
"I hadn't set a target of winning a medal for him at the world championships. But right after the award ceremony, I did wish that he could have been there to see me receive this great fencing honour."
Many of Karen's friends and schoolmates supported her while watching a live broadcast of the competition on YouTube.
"I got lots of messages of congratulations on Facebook and Whatsapp," she says. "I couldn't reply to all the messages, so I wrote a status about how I felt, and thanking people. My coach at the Hong Kong Fencing School congratulated me by simply quoting Lee Lai-shan's famous words, which is why I added them in my comments on Facebook."
Karen, who hopes to be an airline pilot one day, will be unable to defend her Asian and World fencing titles next year because she will be busy studying for her HKDSE exams.
"I know it's hard to balance studies and sport, but my plan is to keep the two things going.
"Apart from junior events, my dream is to compete in the Asian and Olympic Games while I'm studying at university.
"Once I've achieved my sporting goals, I'll be ready to fly passenger jets."
The sky's really the limit!