Dr. Cors works for Green Star Relief to aid the victims of nuclear exposure. He gives his word to Zerchi to not recommend euthanasia to any victims in exchange for being allowed to work in the abbey. But after encountering one hopeless case, he breaks his word and moves his efforts down the highway.
Zerchi and Dr. Cors are both pacifists, and they both want to do what is best for the victims of the nuclear attack. They are very similar characters. So you'd think they'd have a comfy spot on each other's BFF list.
But they strongly disagree on the matter of euthanasia, making them each other's foils. Unlike Zerchi, Dr. Cors doesn't believe in the soul or the eternal punishment of hellfire. Instead, "'pain is the only evil [he knows] about" (27.48).
To Zerchi, the euthanasia law is a "good law" (27.16). He puts more trust in antibiotics than in God (27.50).
It's interesting to note that, unlike Thon Taddeo, Dr. Cors never has a moment when his own foolishness is put on display. The novel paints him as a very forthright man, even if the author isn't necessarily promoting his views.
And even though he breaks his promise to Zerchi, one gets the feeling that he did the right thing. Maybe not by Zerchi's moral standards, but definitely by the standards of his own conscience.
Dr. Cors's name probably comes from the cor- found in such words as corporeal and corpus. Words like these derive from the Latin corporeus, meaning "of the nature of a body" (source). And that fits Dr. Cors very well.
He is a man more concerned with earthly matters like the body and pain. Zerchi, on the other hand, is all about spiritual matters.