2. How to add One to One relation as admin inline?

OneToOneFields can be set as inlines in the same way as a FK. However, only one side of the OneToOneField can be set as the inline model.

You have a HeroAcquaintance model which has a One to one relation to hero like this.:

class HeroAcquaintance(models.Model):
    "Non family contacts of a Hero"
    hero = models.OneToOneField(Hero, on_delete=models.CASCADE)
    ....

You can add this as inline to Hero like this:

class HeroAcquaintanceInline(admin.TabularInline):
    model = HeroAcquaintance


@admin.register(Hero)
class HeroAdmin(admin.ModelAdmin, ExportCsvMixin):
    ...
    inlines = [HeroAcquaintanceInline]