Müller and Messener were involved in an underground drug trade and Johan Liebert had somehow came to know of their illegal activities. Threatening the two police detectives with his incriminating knowledge, he gave them a job: killing the Fortners for him and disappearing for good.
Murder of the Fortners
When Tenma and Nina arrived at the Fortner's household at Necker street, they found Jacob Mauler and the Fortner couple dead. Müller and Messener must have hidden somewhere to let after they had executed their heinous act.
Nina was then groped by shock and horror upon the sight of her dead parents and her recovered memory of a certain 'brother' she had. Tenma helped her stand and tried to take her out for their security.
Then, Müller and Messener showed up when the two were about to open the door. Müller showed them two identification and told them they received a report about an attack. The two police detectives made their way to 'inspect' the house. However, the detectives just stood in front of the crime scene and just commented on the cold-blooded murder.
Messener told Tenma and Nina that their back up is on their way and that they have to take them both for questioning. In their police car, Müller was already asking questions about Nina and about the killer. Although
their facial expressions are out of Tenma's view, it can be seen that Messener was clearly uneasy while Müller was not even disturbed by the multiple murder he just committed.
Tenma then noticed the direction they were heading to, which was away from the Heidelberg police station and the center of the city. Müller told him they were were from Mannheim station. This made Tenma suspect their identities. Tenma recalled that the Fortner's phoneline were cut, so for them receiving a report is an impossibility. Also, they arrived with their police siren silent and neglected their duties on securing the crime scene before taking witnesses into custody. What convinced him is their police badges that looked authentic.
Then, Tenma's suspicions were confirmed by what he saw on the hem Messener's coat: blood. Tenma knew
Müller and Messener were the murderer of the Fortners and Mauler. Near Mannheim, a checkpoint stopped their car. Tenma felt relieved at the sight of other police, but the checkpoint police recognized the two as detectives from Mannheim. Tenma then asked Messner if he could take out Nina for fresh air and he reluctantly agreed.
Tenma and Nina's extended stay out of the car alerted the two detectives. Tenma inquired how did the two detectives knew he was a doctor when all he gave was his name. The two were surprised and started running for them. Tenma then moved on instinct and threw himself and Nina to the river to escape the detectives.
Müller recieved a huge sum of money from Johan after he murdered the Fortners. He built a large mansion in a small port city Nice in the south of France. He intended to live there a peaceful life with his wife Barbara and his son Fritz. However such an easy way, he made a money and living under the eye of the unknown organization have been hunting him ever since he moved to France. He told that this keeps him bugging to Roberto (whom he considers reliable). Müller hired a private detective to learn more about the incident that occupied in Düsseldorf related to Tenma, 10 years ago, Johan and Nina Fortner. Müller also learned then that his partner Messener has been stabbed to death. Shortly after Müller has received the information, the detective has been found dead, apparently attacked by the unknown figure, later revealed as Nina. She told Müller that Roberto killed him and the two escaped before he appeared.
Nina threatened Müller for the death of her foster parents and willed to kill him. He said that he and Messener were blackmailed and that they had no choice. Müller repented for what he has done and started crying. Nina gave up on killing him.
In a meanwhile, Roberto called and says that he wants the documents containing data related to Johan, he also said that his family is relaxing there with him. Müller reacted panicked and along with Nina, headed back to his house. The duo armed themselves and separated ways. He gave Roberto the bag with documents, he also noticed a couple of hitmen and a vanished body of the deceased detective. Roberto and his people left the house. Müller followed him which led to an abandoned building where they kept Nina. Roberto said Johan's location and left. His hitmen were about to kill her but failed when Müller appeared and shot them all. One, however was still movable and shot Müller in the side, causing him to bleed profusely. Nina tried to drive him to the hospital, though it was in vain, as he expired from his injuries, as confirmed in Another Monster. Later on, when Roberto chats with Eva in order to ultimately pursue Tenma, he mentions that the security company in Nice had collapsed due to the death of a client. Whether he is referring to Müller is unclear.
Muller hired him as a bodyguard. In reality, Roberto was sent by Johan to kill him.
He seems to have a good partnership with Messener and was visibly shocked when he heard about his death.
Barbara and Fritz
Wife and step-son.
In Chapter 6, it is confirmed he died when he was shot by Roberto's men.
- "Roberto, who are... who are you?"