The male MC disappeared and there is no trace of him, is there any way to return him to base? I tried to search in the guide but I didn't find anything about it.
If you got a blue flag, interact with it an press "Call", this should call all special NPC's including your other Main Character to the flag.
After which, ensure you set his position at the desires spot for him to stay at (by default he'd always return to the beach, or attempt to do so atleast)
Alternatively you can also press ENTER and type /change into the box, which should make you swap characters.