Ma, whose delicate frame and shy smile makes her an unlikely warrior in real life, uses Google in her spare time to search for blogs, videos and other Internet postings that identify fellow South Koreans who are "sincerely idolizing" communist North Korea.
In the past year, Ma has reported more than 30 online postings that she considered dangerous and which could "brainwash the minds of South Koreans" to the National Intelligence Service. Using search keywords such as "Great Leader" or "Nation's Sun" - references to the North's dynastic leadership - she trawls the Internet until finding offending content and then submits a link along with a screenshot online.
The agency uses the tips to investigate possible violations of South Korea's National Security Act which prohibits praising or glorifying North Korea - a crime punishable by up to seven years in prison. North and South Korea technically remain at war, having never signed a peace treaty after their 1950-53 conflict ended with a still uneasy truce.
The number of prosecutions under the security law, which are not limited to glorifying North Korea, has risen in recent years. Convictions are harder to secure but not uncommon. Last September, a 43-year-old South Korean man was sentenced to 10 months prison for operating a pro-North Korean website and posting over 300 messages and 6 videos praising North Korea.
The zealous policing of the Internet by a rising movement of young nationalists seems at odds with the wealthy, dynamic and democratic image that South Korea projects to the outside world. Yet for South Koreans it is unremarkable, even if not accepted in all corners of society. Anti-communist instruction was a staple of the curriculum in South Korea's schools into the 1980s. Liberal groups say similar anti-North Korea education has been revived under conservative President Lee Myung-bak, although the education ministry disputes that.
Activists like Ma see themselves as cyber guardians of national security, keeping the Internet in South Korea safe from infiltration by pro-North Korean ideas. In doing so, they have become a vigorous vanguard for enforcement of the national security law, which critics argue puts excessive limits on freedom of expression.
The young activists are a "manifestation of misleading patriotism," said Park Lae-goon, director of Human Rights Foundation Saram.
"Under the anti-communism environment, South Korean society has denied diverse values," Park said. "In a democratic society, we should be able to talk about different ideologies and opinions."
The National Intelligence Service sees it differently and rewards informants with a metal watch that has become a coveted item among teenagers. They call it "the One Watch," after "the One Ring" in the popular "The Lord of the Rings" movie trilogy.
Ma and another activist Park In-beom, 17, both received the One Watch. Park said he was extremely happy when he got the gift that came with a thank you letter from the spy agency.
"I realized I was doing the right thing," he said.
Many of the activists say it was North Korea's shelling of a South Korean island in 2010 that catalyzed their anger and mobilized them. It was the first assault by the North on a South Korean civilian area since the Korean War, singling it out from North Korea's other deadly provocations over the years.
At a recent retreat near Seoul attended by about 40 online campaigners, Park In-beom recalled that the attack made "his blood boil" and his body "shiver with indignation."
"I seriously contemplated what I can do for my country. Because I'm not an adult, I can contribute by reporting the posts that threaten national security in the cyber world," he said.
In summer or winter vacations, he goes online for two to three hours a day, wading through web page after web page looking for North Korea sympathizers. For each spell online, he would usually report three to four cases to the intelligence agency. He wants to work for the government in the area of national defense.
It is unclear if there is a link between the Internet patrolling and the rise in prosecutions or increased government censorship of the Internet. The amount of pro-North Korea commentary on the Internet and the level of censorship waxes and wanes with the degree of tension between the two nations.
The conservative government that took power in 2008 has taken a harder line against North Korea sympathizers. Last year it blocked access to 187 pro-North Korean accounts on Twitter and other social networking services. It also removed 79,038 online posts that glorified North Korea, about a 40-fold increase from 2008.
"Cyber activities undermining national security have been increasing but what the state can do about them is limited," said Yoo Dong-ryul, a senior researcher at the conservative Police Science Institute, a state-run think tank. The teenage activists are part of a "self-cleansing process without state intervention."
The Supreme Prosecutors Office said it received 127 recommendations to prosecute alleged violations of the security law last year, double the number in 2009. The number of suspects indicted rose from 43 to 63 in the same period.
In 2010, 52 people were indicted by prosecutors. Some 20 were found guilty and seven received prison sentences, according to Lee Yong-kyung, a former lawmaker of a minor opposition party, who received the figures from the Ministry of Justice.
It's not known how many of those prison sentences were for glorifying North Korea. The justice ministry and the Supreme Prosecutors' Office didn't respond to requests for details.
A focal point for the activists is Naver, South Korea's largest web portal, which hosts three online communities dedicated to weeding out pro-North Korean sentiment. All have started in the past one to two years.
The "Blue Eyes" community was launched following North Korea's artillery attack on Yeonpyeong island and attracted over 4,000 members with over half under 30, said its manager Jang Mun-jun. Another online group started by a 15-year-old attracted nearly 1,000 members, mostly teens, in a little over a year.
Members proudly share the screen images of pro-North Korean content they have submitted to the intelligence agency and encourage each other.
These have also made inroads into Twitter to counter left-wing pundits advancing reconciliation with the North. The microblogging service has become popular among South Korean liberals who are critical of President Lee Myung-bak.
"Twitter is not a tool for communication but a weapon," said one 16-year-old student who had reported more than 100 cases of pro-North Korean online content to the intelligence agency. "Joining Twitter is a must-do for patriotic activities." The student requested anonymity, citing the possibility of becoming a target for pro-North Korea activists who might include teachers.
Kim Jong-bo, a member of Lawyers for a Democratic Society, said that having grown up with anti-communism education, he can understand why students follow the government without questioning.
"I'd like to tell them to listen to different opinions," Kim said.