2009–2011: Superstar K and music releases In 2009, Seo competed in the
Mnet singing competition program Superstar K, winning the show's first season. On October 27, 2009, he released his debut
extended play Calling, with the same-title lead single composed by
Bang Si-hyuk topping multiple music sites on its release day. Seo signed with
Jellyfish Entertainment in the same year and won the Male Newcomer of the Year Award at the 2009
Cyworld Digital Music Awards. In May 2010, Seo released his first single album
Just Beginning with the lead single "I Love U". Seo made a brief foray into acting for the single's
music video, to which he described his first acting experience as "awkward", saying, "I felt my hands and feet shrivel." The album reached number seven on the
Gaon Album Chart, while the single peaked at number two on the
Gaon Digital Chart. A repackage edition of the album titled
My Baby U was released three months later. Having lost over 33 pounds (15 kg) since his debut through exercise and strict diet control, Seo's changes in his physical appearance brought about rumors of
plastic surgery. In 2011, shifting his sound, Seo released the single "Broken" incorporating choreography into his performance, as well as summer dance track "Shake It Up" in which he penned the lyrics for. Despite a successful debut, Seo reportedly received the "
cold shoulder" treatment from the three major Korean public networks, as he was a cable television star.
2012: Acting debut and Reply 1997 In 2012, Seo was part of the cast of the
jukebox musical Gwanghwamun Love Song and made his onscreen acting debut with a supporting role in
KBS's
Love Rain. According to Seo, his role in
Love Rain came at a time when he was feeling anxious about the life of an entertainer, and fueled his desire for acting. He landed his first leading role later that year, in cable channel
tvN's '90s nostalgic hit drama
Reply 1997. Seo's portrayal of a teenage boy in unrequited love with his childhood best friend won praise from both audiences and critics.
Yonhap News praised, "It's hard to believe he's a rookie, from acting in his eyes, to handling lines and expressing emotions". The first duet "All For You", a
remake of the song by
Cool, topped weekly and monthly music charts and became one of the best-selling singles of the year on the Gaon Digital Chart. The single also won the Best OST award at the
Melon Music Awards and
Mnet Asian Music Awards that year. and also signed with Japanese agency
Irving Entertainment.
2013–2015: Japanese debut, variety shows and return to acting Seo joined the newly launched reality/variety show
I Live Alone from January to June 2013. He returned to the music scene in April 2013 with the single
With Laughter or with Tears, the first soulful
ballad from his discography. Several weeks later, the song was followed by his Japanese debut with the release of the single
Fly Away.In June 2013, he was cast in ''
Master's Sun, a drama series written by the Hong sisters, playing a former soldier who served in the Zaytun Division who becomes the head of security for a shopping mall company. In October 2013, he starred in his first big-screen leading role in No Breathing, a coming-of-age film about two swimming rivals, alongside Lee Jong-suk and Kwon Yuri. He then played a dual role in Another Parting'', a five-episode
miniseries about a man and a woman who meet in the final moment of their lives and spend one special day together. In 2014, Seo released his first Japanese album titled
Everlasting, which comprises his previously released singles "Fly Away" and "We Can Dance Tonight". The album also features a new track titled "Everlasting Love" with a total of 12 tracks. The album debuted at number nine on the
Oricon Daily Chart. Seo later won the Best 3 New Artists (Asia) award at the
Japan Gold Disc Awards. Seo headlined cable romantic comedy series
High School King of Savvy in which he played a high school student and star hockey player who leads a double life when he's forced to take on his elder brother's job as an executive at an
IT company. Later that year, Seo starred in his first
period drama ''
The King's Face'', playing illegitimate crown prince
Gwanghae, who becomes his father
King Seonjo's rival in politics and love. In 2015, Seo joined the cast of
SBS variety show
Law of the Jungle from March to May, for the Indochina season. He was then cast as a genius
profiler in the
police procedural/romance series
Hello Monster which premiered in June 2015 on KBS2.
2016–2020: Career fluctuations In 2016 Seo joined
Law of the Jungle for the second time for the show's Mongolia season. He also starred in
OCN's workplace drama
Squad 38, playing a professional swindler. Seo called the role a turning point in his acting career, and he received favourable press reviews for his ease of transformation into multiple character personas. The drama became the highest rated drama for OCN, and Seo won a special award for his role at the 11th Asian TV Drama Conference. He then took on the leading role in
MBC's
romantic comedy Shopping King Louie, playing the role of an
amnesic chaebol heir with childlike innocence. For his performance, Seo was awarded the Excellence Award at the
2016 MBC Drama Awards. On August 7, it was announced Seo had signed with BS Company. In 2018, Seo starred in mystery
melodrama The Smile Has Left Your Eyes, a remake of the 2002 Japanese television series
Sora Kara Furu Ichioku no Hoshi. The drama reunited Seo with director Yoo Je-won, whom he had previously worked with in
High School King of Savvy. Seo had initially contemplated taking up the role but decided to join the project following numerous discussions with director Yoo. Despite the drama's lackluster ratings, Seo's performance garnered positive press reviews.
The Straits Times wrote, "It is a role that could easily slip into creepiness or smugness but Seo manages to pull off a delicate balancing act." Seo was set to star in film
Hip Daddy but stepped down from the project in 2020 after production was put on hold. In May 2021, Seo made his small-screen return with fantasy romance drama
Doom at Your Service as the
personification of destruction. In the same month, he also returned to the big screen after eight years in
Yoo Ha's comedy-crime film
Pipeline as a drilling expert involved in an oil
heist. Director Yoo said about Seo's casting, "I was fascinated the first time I saw him. [...] His face showed so much different potential."
2021–present: Return to music and ''Death's Game'' In December 2021, Seo signed a singer contract with AER Music. On June 14, 2022, Seo released his new single album
Love&Love and its lead single "My Love", featuring
Ravi, his first release after five years. A teaser for the music video was released on June 9, 2022, on YouTube. The same year, he played a
profiler-turned-
shaman in
KBS2 drama
Café Minamdang, alongside
Oh Yeon-seo. On June 14, it was announced that Seo 'Heart & Love' fan meeting will be held at Yes24 Live Hall on July 30. On December 6, 2022, Seo will release the single "Fallen", the first single released in six months. Seo then starred in Kim Hong-sun's action thriller film
Project Wolf Hunting as a criminal with
Jang Dong-yoon, alongside Choi Gwi-hwa and
Sung Dong-il. On February 8, 2023, Seo was confirmed to star in the
TVING original series ''
Death's Game''. ==Personal life==