Halsin has been a companion. So has Minthara. But they are not origin characters. Even in Larian's email, they make that distinction. I believe the distinction is that the companions will join our groups for certain missions where we may need them (clearing the goblin camp or attacking the druids), and as long as we have aligned ourselves with them. But they won't be like origin characters where we can swap them out for any and all questing.