Doctor J. W. Müller (French: Docteur Müller), also known as Mull Pasha or just Müller, was a psychiatrist whose position and credentials serve more as a cover for his more criminal activities, including that of a criminal, secret agent and mercenary for hire.
Müller's first appearance was in The Black Island where he was the mastermind of a counterfeiting ring. He located himself inside an English mansion and practised as a doctor as a cover front for his operations. Once Tintin discovered this, he and Müller had a confrontation which resulted in the manor burning to the ground. He then retreated to his base of operations underneath Craig Dhui Castle, near Kiltoch in Scotland. Tintin pursued and managed to expose the counterfitting ring where Müller and his cronies were arrested.
He also appeared in Land of Black Gold and The Red Sea Sharks. In the later two books, he aided the Arab rebel Bab El Ehr in his efforts to remove the Emir of Khemed, Mohammed Ben Kalish Ezab. He designed Formula Fourteen, a chemical substance which increases the explosive properties of petrol, and incapacitated the pipelines of Arabex, the Emir's favoured fuel company.
By the time of The Red Sea Sharks, Müller changed his name to Mull Pasha, as seen in the stack of tabloid clippings near the end of the book. He ordered Tintin and Captain Haddock's deaths but after a grammatical mistake with Colonel Achmed, they manage to blow up their own tanks searching forTintin as he travels to the Emir.
In Land of Black Gold, upon the discovery of Formula 14 by Tintin, he pulls another pistol from his jacket and holds it to his head, attempted to commit suicide, but when he pulls the trigger, ink squirts out, because it was rigged by the Emir Mohammed ben Khalish Ezab's son, Abdullah.
In The Black Island, he has a goatee and moustache, but by the later two books in which he appears, he had grown a full beard, permitting him to easily disguise himself as an Arab. Once Bab el Ehr is overthrown at the end of The Red Sea Sharks, he is also captured and presumably executed or incarcerated after a shootout with both Tintin and Captain Haddock.