Doctor Krollspell (French: Docteur Krollspell) appears in Flight 714 and Tintin and Alph-Art. He is an associate of Tintin's enemy Rastapopoulos, but he later changes sides when it is in his interest to get away from his employer.
It's been suggested that Krollspell is an ex-Nazi scientist, probably based on Josef Mengele, the infamous "Angel of Death," or Adolf Hitler's quack doctor, Theodor Morell. In an interview, Hergé himself suggested that Krollspell had worked in a concentration camp despite Flight 714 to Sydney having been published over 20 years after the war.
"Kroll" is also part of the name of the Krolloper Berlin Opera House, where the Nazi-dominated German parliament met following the Reichstag fire of 1933. However, "Krolspel" is simply Brussels dialect for "krulspeld", which means "hair curler".
Krollspell was the head of a psychiatric clinic in New Delhi (Cairo in the English version of the story). He developed a truth serum that Rastapopoulos intended to use on kidnapped millionaire Laszlo Carreidas in order to find out the number of a Swiss bank account, where Carreidas had left a large part of his personal fortune under a false name and signature, presumably for taxation purposes.
The corrupt doctor injected the millionaire with the serum. Carreidas proved more than willing to tell the truth—but about everything except the Swiss bank account. To Rastapopoulos's fury, Carreidas launched into long disquisitions about his greedy, unscrupulous nature, boasting about how he first stole a pear at the age of four, framed the family maid after robbing from his sister's handbag, shamed his great-aunt to death, and had generally led a life of perfidy. Realizing the serum was defective, a furious Rastapopoulos lashed out at Krollspell, who was still holding the truth-drug syringe, and was accidentally injected with it, becoming intoxicated as well. Rastapopoulos now also recounted his hideous deeds in a boasting manner, calling himself the devil incarnate. He even revealed that he intended to double-cross and murder all his associates, including Krollspell, apart from Allan, and not pay him the $40,000 he had promised him.
Unnerved by these revelations, Krollspell was about to escape when he was captured by Tintin and Captain Haddock, who had come to rescue Carreidas. He and the two drug-induced men were then tied up and gagged. However, when the serum wore off, Rastapopoulos made an attempt to escape, and Krollspell was quick to warn Tintin and Haddock. Rastapopoulos got away, but the doctor was released and continued to accompany Tintin and Haddock, watching over the still-irritable Carreidas. Haddock only grudgingly went along with Tintin's release of Krollspell, as the good captain had a tendency of treating even reformed enemies, such as Frank Wolff, with suspicion, although Tintin pointed out that the news of Rastapopoulos's treachery gave Krollspell every reason to help them escape.
Krollspell, along with Tintin and his other companions, was later picked up by a flying saucer. A treatment by the aliens caused him to lose his memory completely about his time on the island. In a news program later in the story, it is announced that Krollspell was found in Cairo with no memory of how he got there. In Alph-Art he is director of a brown sugar factory.