I've seen in many streams that Isaac has D6 from the very beginning. How can I get it? P. S. I have all characters except Magda.
As noted in the Rebirth Wiki, which is actually correct in this case, the D6 is unlocked by beating the Cathedral as Blue Baby/??? (who in turn in unlocked by beating Mom's Heart 10 times).
Once it has been unlocked, Isaac will carry it as a starting item.