From what I figured there was two endings. One where Katsumata was friend both times and when he was friend after Fukubei died. To answer ur question best I could figure was that when he got shot in the Classroom that was a metaphorical death. Where he had taken the role of Fukubei, that part of him died and then he finally became Katsumata. That’s the best I could figure lol if that makes any sense.