Detective Messener is a look-out during the murder of the Fortners He was Detective Müller's partner until they were ordered to murder the Fortner couple by an anonymous man who knew about their drug trading
Detective Messener lived in Munich, working as a police detective. His partner was Michael Müller. Because they were both at the edge of downfall, the duo became involved in an illegal drug trading. Yet they were scraping with those jobs dangerously. Then there appeared an unknown man who knew of these actions.
That anonymous man started blackmailing the two partners by threatening them; if they don't do one very awful job, they would end up being arrested by police. That mission was to assassinate Nina's foster parents, the Fortners, in Heidelberg. Seeing that they had no choice, they were forced to agree with it.
Tenma found the bodies of the Fortners and Jacob Mauler in the Fortners' home. After that, Messener and Müller arrived at the address, claiming that they were called by other neighbors to see what happened. Tenma believed them once they had shown their real police badges, and he and Nina left the house with them, heading to a "police station". Tenma became very suspicious when saw blood on one of their coats. Messener was very upset about the terrible crime they had done, but Müller was able to stay calm. Tenma asked them if they could let him and Nina to get some fresh air at when they were stopped by a roadblock.
Tenma realized that the bridge they were stopped at, was above a river, and saw this as a chance to escape. He decided to through with it when one of the men referred to him as "Doctor" Tenma, despite the fact that he never told him he was a Doctor. Tenma and Nina jumped off the bridge into the river and the partners were unable to chase them.
Tenma later confronts Messener, who initially believes Tenma to be his drug dealer. Tenma notices he is showing withdrawal symptoms, and says that he has used all of his reward money for killing the Fortners on drugs, where as Müller used his money to live comfortably with his family. Desperate for Tenma to leave, he tells Tenma about his involvement with a man known as The Baby. Tenma also learns that Nina had also confronted Messener for similar information.
A few months later, Müller learned that Messener was stabbed to death in a drug trading in Frankfurt. Yet a detective hired by Müller found that Messener was not in fact killed in a drug trade gone bad, but by a man that the Detective's notes referred to as Roberto.