Amuro (Rei Furuya) inwardly reflects on Scotch’s death, maintaining his bitter belief that Shuichi Akai was the one who killed him—a core conflict that drives much of their rivalry in the series. The Unfriendly Girls Band ~ Part 2 - Detective Conan Wiki
It is confirmed that the friend was Scotch , a member of the Tokyo Metropolitan Police Public Security Bureau who infiltrated the Black Organization alongside Amuro. Detective Conan Episode 837
The case triggers a flashback for Masumi Sera. She remembers seeing her brother, Shuichi Akai , at a train station four years prior with a mysterious friend who taught her a few notes on the bass guitar. Amuro (Rei Furuya) inwardly reflects on Scotch’s death,
Rumi used a cell phone recording trick to manipulate the timeline of the murder. By filming vertically, she was able to hide specific movements while playing a recorded keyboard track to make it seem as though Hagie was still alive and sleeping during the rehearsal. She remembers seeing her brother, Shuichi Akai ,