Billy is a nickname of the bassist of the band called Mad Mouse with Kenji as the lead singer and guitarist, and Haru Namio as the drummer. The band disbanded after Haru Namio left the band. In the 3rd Friend year, Billy owned a Yakitori restaurant before Haru appeared to tell him that Kenji is coming back, and then they practiced their skills before performing music again in the Expo Woodstock festival along with Kenji.

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