The phrase "main composer" refers to the person who creates the music and is responsible for writing the majority of the songs or pieces. Full definition
Turning down the original series main composer, the person responsible for near enough every memorable bit of the original series soundtrack? (gamingreinvented.com)
Oceanhorn: Monster of Uncharted Seas made a name for itself when it was released all the way back in November 2013 on iOS devices, for not only being a very impressive looking title at the time, but also for being endorsed by big names of the industry like Kenji Ito, main composer of Squaresoft's SaGa franchise, and Nobuo Uematsu of Final Fantasy fame, both of whom composed a few music tracks for it. (cubed3.com)
Series veterans are returning in their usual capacity, specifically Katsura Hashino as director, Shigenori Soejima as character designer, and Shouji Meguro as main composer. (randomnpc.com)