Career beginnings (2008–2015) Koo made his acting debut in the 2008 short film
Boys, directed by his
Seoul Institute of the Arts peer
Yoon Sung-hyun. In the film, he portrayed a high school student with a specialized interest in kite-making. He began to garner attention for both his acting and directing after winning the Ting-Geurang Coin Award at the Jeongdongjin Independent Film Festival for his 2011 film
Turtles, which he also directed. In 2014, he served as the director, screenwriter, producer, editor, and lead actor for the segment "Dating Documentary" within omnibus film
Now Playing, the opening feature of the Seoul Independent Film Festival.
Critical recognition in independent films (2016–2019) In 2016, Koo starred alongside
Lee Min-ji in Cho Hyun-hoon's independent
film Jane. He portrayed the title character, a transgender woman. To prepare for the role, Koo lost 10 kilograms and developed a distinctive high-pitched vocal delivery. His performance earned him Actor of the Year at the
Busan International Film Festival and the Best New Actor Award at the 54th Baeksang Arts Awards. That same year, he won the Grand Prize in the Domestic Competition at the Asiana International Short Film Festival for
Fly to the Sky, a project he co-directed and starred in. Koo continued to establish a unique presence in the industry through his collaborations with director Lee Ok-seop. For the 2018 film
Maggie, he served as the producer, screenwriter, editor, and lead actor. The film received significant acclaim, winning the Audience Award at the 44th Seoul Independent Film Festival, the CGV Art House Award at the Busan International Film Festival, and the KBS Independent Film Award.
Transition to commercial film (2020–Present) Koo's first major role in a commercial production was Captain Seo in
Yeon Sang-ho's 2020 film
Peninsula. The director initially doubted Koo's suitability for commercial cinema but cast him after a personal meeting. Yeon later noted that Koo's interpretation added complexity to a character that might otherwise have been a conventional villain. His performance in the blockbuster garnered significant public and critical attention. ==Personal life==