This was followed by major roles in period drama Diary of a Night Watchman (2014), thriller Last (2015), and fantasy Moorim School: Saga of the Brave (2016).

She also appeared in the music video of South Korean boy band Big Bang titled "Let's Not Fall In Love" (2015).

[10][11] In 2016, she played one of the main characters in the KBS drama Moorim School: Saga of the Brave (2016) and marked next big-screen project with the film Seondal: The Man Who Sells the River (2016) in a supporting role.

[27] She starred opposite Kim Soo-hyun and featured in the role of a bestselling children's book author who has a personality disorder.

[28] Her portrayal of the character was critically acclaimed by Manila Bulletin,[29] The Hindu,[30] and The New York Times named the series one of "The Best International Shows of 2020" and called Seo's performance as "mesmerizing".

[39] In April 2021, Seo was accused of being the reason behind Kim Jung-hyun's behavior at a 2018 press conference for The Time and his exit from the series.

[41] In response, Seo's agency, Gold Medalist, denied the claim that she was the reason behind Kim's behaviour and abrupt exit from the series.

They argued that "it is unreasonable to assume that a lead actor would make a decision that would jeopardize his career without free will", adding that they "confirmed" this with Kim Jung-hyun.

[43] In May 2021, Kim Jung-hyun revealed that he had ended his contract with his agency, whom he accused of forcing the 2018 press conference on him despite his health problems.

In addition, Kim also denied Seo has any involvement in how he acted, reiterating the issues on his health problems and company's mistreatment.