Leo Naphta is the novel's second pedagogue: a teacher of ancient languages, a former student at a Jesuit school and a member of the Society of Jesus, who enters in chapter VI as Settembrini's adversary and defends faith, authority, the City of God, and the "necessity of terror".
He lives in the village, in a tailor's house, as Settembrini's neighbor, and the text describes him entirely in angles: hooked nose, thin lips, thick glasses. His biography explains the violence of his ideas: the son of a Jewish ritual butcher who died crucified in a riot, he was taken in by a Jesuit, converted, and only failed to become a priest because illness stopped him at the subdiaconate.
In the debates, he denies that there is any knowledge neutral to faith, with Saint Augustine's "I believe so that I may understand," and he holds that "the sicker, the more human." He does not even suspend the dispute before Joachim's body. In chapter VII, as his health worsens, he takes the quarrel with Settembrini to a duel; when his opponent fires into the air, Naphta calls him a coward and shoots himself in the head.
The novel does not defeat him by argument. The text narrates Naphta's suicide without giving him a reason.