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I can confirm BrokenGrunty's method with a caveat. You can get Bellatrix (Azkaban), but you have to wait until 7.2 - 2 to do it. That's when you return to Hogwarts. That's the soonest Dark Wizard\Witch I was able to get.
I hope someone can confirm this. I decided to test out a theory that you can use Salizar Slytherin as a Dark Wizard. I was already at 100% at the time, and he worked out. Then I started over, trying to figure out the earliest you can gather a Dark Wizard character. Just as soon as I finished the DADA class (Where Umbridge replaces the standard DADA book with the kitten and ball of yarn book), which is when you lose DADA spells, but first are able to access the spell wheel, I went back to Diagon Alley. I started the first level in freeplay with Salizar Slytherin, but was unable to free the student in peril from the dark cage in the drainage culvert. Every magically movable object (Wingardium Leviosa) highlights red, not purple as most wizards\witches do, so I don't know why I couldn't free the student. Perhaps you can't access Dark Wizard abilities until late in the game? Anyone who can confirm, please do.