“Mulder” is set to become a companion in the 0x38 chapter Frelia castle’s opening event.
Therefore, you need to change it.
(The opening event of Frelia Castle is interesting and it will be executed even on a new journey(After Chapter 9 clear).)
Once a unit becomes a joined the class is used when you become a join for the first time.
This is the specification of the FE program.
0x38 At the opening event of Frelia castle, because Mulder is there, I changed his class to Berserker(バーサーカー).
It is working properly.