Yoo Si-jin is a fictional character from the popular South Korean television series Descendants of the Sun. He is portrayed by actor Song Joong-ki and serves as a captain in the South Korean Special Forces. The character is known for his bravery, leadership skills, and dedication to his country. In the series, Yoo Si-jin falls in love with Kang Mo-yeon, a skilled surgeon played by Song Hye-kyo. Their relationship faces various challenges due to his military duties and her medical career. The show combines elements of romance, action, and drama, making it a hit both domestically and internationally.