Of 2,333 adult respondents in recruitment website Incruit's annual survey, 20.9 percent picked the penguin character named Pengsoo, compared to 17.6 percent for BTS.
It marks the first time a character has topped the list, according to Incruit.
The expression-less giant penguin with a husky voice was created by EBS, South Korea's public educational broadcaster, to target teenagers.
But Pengsoo has drawn many adult fans, as well, gaining fans in their 20s and 30s with its straightforward attitude and blunt personality.
Although it is owned by EBS, Pengsoo regularly appears on programs of other TV stations, which is unusual.
Its YouTube channel, Giant PengTV, has attracted more than 1 million subscribers in about six months to earn the Gold Play Button awarded by the video-sharing platform.
Fans eagerly attend Pengsoo's meet-and-greet events across the country. Its essay collection has become a bestseller.
Companies including clothing brands and snack suppliers are lining up to feature Pengsoo, which has a nickname of "president of workers," in their commercials.
The Ministry of Foreign Affairs asked the penguin to promote a special summit last month between South Korea and the 10-member Association of Southeast Asian Nations.
Some attribute Pengsoo's popularity among young people to the character's ability to reflect their lives, struggling to find regular jobs and decent housing.
"While working like interns, Pensoo sleeps at a small room of the EBS building. Many millennials appear to love Pengsoo because they can easily sympathize with it," cultural commentator Kim Seong-soo said.