Heinrich Kramer (not Kemmler) was a german inquisitor (also known by his Latinised name, Heinricus Insitor). He widely distributed the Malleus Maleficarum - literally "the Hammer of the Witches" (1487), which described witchcraft and endorses detailed processes for the extermination of witches.
He was also asked by a Nuremburg council to provide expert consultation on a witch trial.
He was the rough basis of the Warhammer Fantasy character, Heinrich Kemmler, from who Jim has named his own character as a homage.
Heinrich Kemmler in the Dresden Files is a body-swapping, bad ass necromancer who was so feared and despised that he took on the entire White Council and quite nearly beat them. He engineered WW1 and possibly WW2, and seemed to have many unsavoury associations with demons, vampires (particularly Black Court) and the "nastier" faeries. Mab describes him as mad but brilliant, Leansidhe regards him highly and even his own heirs (acolytes) respected and feared him, even if some of them even thought he was mad.
He is one of the few mortals to work out how to eat spirits to become stronger, and eventually figured out how to become a god through that (although he is far from the first mortal to figure out how to become a god altogether). He evaded the White Council for decades and it took the several times to kill him (who knows why they are so sure he is really dead this time, but apparently they are). He seemed to have some business in Illinois just before WW1 and quite possibly was responsible for the Spanish Flu (according to a recent but cryptic WOJ) and Wyatt Earp seemed to cause him some trouble too. He seems to be linked to all the worst sorts, and I am sure he is linked to Outsiders (which, like his name sake, would be a source of his power).
Could Kramer and Kemmler be the same person? I think this is quite possible. I guarantee Jim would know of the connection, but will he use it?
And how does an inquisition priest become a mad, power hungry necromancer?