Wait...there are characters you can put into your party that aren't Origin characters other than the newly Announced Mercs (Jaheria, Minsc, Halsin) ?
I suspect you are confusing things. Which is OK< as BG3 is confusing

Origins - the Original Five, Karlach and Edgelord - you can play as one of them and take on the rest into the party, available practically immediately
Companions - Jaheira, Minsc, Jaheira, Halsin - there become available later - there probably are some differences in how they fit into the party versus Origins
Mercenaries/Hirelings - another separate category, closer to Origins, but without an Over The Top Tragic Past and Sob! Backstory. Almost certainly available very quickly, just after Origins, thus much earlier than Companions
I may be confused myself

hence treat my revelations with caution :P