Kōsuke Miyauchi is a Japanese musician and voice actor, born on December 22, 1986, in Miyazaki Prefecture. His career began in music, starting drum lessons at the age of ten. He joined the rock band Lillies and Remains in 2009, performing at international festivals such as The Great Escape and Liverpool Sound City. After leaving the band, he traveled to the United States to study drums under Billy Martin of Medeski, Martin & Wood. His work as a drummer has included session and live support for a variety of artists.

Miyauchi's career as a voice actor began in December 2019 when he joined the mixed-media project ARGONAVIS from BanG Dream! as a member of the band GYROAXIA. Within the project, he provides the voice for the character Miyuki Sakaigawa. He has voiced this role across all related media, including the television anime series Argonavis from BanG Dream! which aired in 2020. He also reprised the role in the theatrical films Gekijōban Argonavis: Ryūsei no Obligato in 2021 and Gekijō-ban Argonavis Axia in 2023. Beyond animation, he has played Sakaigawa in the stage play adaptations of the series and in the project's smartphone games, Argonavis from BanG Dream! AAside and Argonavis -Kimi ga Mita Stage e-. As a member of GYROAXIA, the band made their major label debut under Universal Music's Virgin Music label with the release of the mini-album Freestyle in February 2022.

As the only member of GYROAXIA with prior professional band experience, Miyauchi serves as both a technical and emotional pillar for the group. Having had no acting experience prior to this role, his approach to voice acting involves finding common points between himself and the character's values and way of life. He left his agency, Ace Crew Entertainment, at the end of April 2022 and has since been working as a freelancer.

Outside of his music and voice acting work, Miyauchi is an avid angler. He is a member of a fishing group sponsored by a tackle manufacturer and frequently competes in local bass fishing tournaments, achieving victories. In April 2023, he caught a record-breaking largemouth bass at Lake Kameyama, measuring 66 centimeters and weighing 5,355 grams.