Fortitude (Hangul:불굴; Romanized: Bulgul) is a song made for Arknights that is performed by the Zainichi Korean composer Yang Bang-ean/Kunihiko Ryo and the violinist Echae Kang. It is later enlisted in the second part of his album, "Shadow," in his 2021 work Light and Shadow.
The song is dedicated to Amiya in her Guard form as a commemoration of the release of Episode 08 on the KR server as the first Thank-you Celebration. It is also considered the first song made for the KR server.