Since 2005, he has been affiliated with Keiichi Okabe's music production company Monaca, where he has often collaborated with his colleagues to produce soundtracks for anime and other media.
Director Yutaka Yamamoto, who was also at the same university, asked Kōsaki to compose the soundtrack for his film Onnen Sentai Ressentiment, as well as playing the role of Emperor Komuro.
Series production director Yamamoto recommended Kōsaki as the composer for the anime, who found it difficult to get involved in such a large scale project outside of Namco, serving as another reason for quitting the company to join Monaca.
[9] An album containing three of his compositions ("God knows...", "Lost my music", and "Koi no Mikuru Densetsu") for the anime sold more than 136,000 copies in Japan and peaked at #5 on the weekly Oricon Singles Chart.
Although he often incorporated live musicians in his previous Namco work, this served as the first time he composed a track fully performed by a band.
[12] He found the workload overwhelming, as prior to joining Monaca he was not used to composing a large number of tracks under time constraints.
The song "Renai Circulation", features lyrics by Meg Rock and vocals by Kana Hanazawa as character Nadeko Sengoku.
[17] In 2020, a compilation album titled Satoru Kosaki 20th Anniversary Selected Works "DAWN" was released to commemorate 20 years of being a composer.
He aimed to compose powerful songs to correspond with the story and imagery, and has been happy to see his work receive such high praise.