You can't leave the main character in camp, because supposedly that character is your personal agent and avatar in the game. Currently you can only have a custom character as your main character, but going forward you would be able to choose one of the origin characters as your main one, forgoing any need for a custom character at all.
I understand this, we are also supposed to be able to make an entire custom party eventually as development continues. I thought that when I mentioned the host character not being able to change their character it implied that the main character could not change. The option I want is only for when using an entire custom party to be able to leave at least one of them in camp.