Anna Liebert

Birthday: April 7, 1975 Johan's twin sister and the only unharmed survivor of the night when both her parents and her brother were shot in what appeared to be a botched burglary. At first she showed signs of amnesia due to the psychological trauma of the incident. After she and her brother disappeared, she was adopted by the Fortner family from Heidelberg, who were unaware of her previous identity as Anna Liebert. As Nina Fortner, she was a hardworking law student at an unnamed university (likely the University of Heidelberg), as well as an adept practitioner of Aikido. She lived her life in peace until Johan made contact with her on her twentieth birthday. She comes to pursue Johan, albeit by different methods and for a different reason than Tenma.

Kenzou Tenma

Doctor Kenzou Tenma is a Japanese neurosurgeon working at the Eisler Memorial Hospital in Düsseldorf. At the beginning of Monster he is favored by the department director for his prodigious surgical skill. After a crisis of conscience, he chooses to save the life of a young boy instead of the city's mayor, and is unjustly demoted as a result; he then becomes suspect for murder when the department director and several prominent doctors are killed weeks later. It is only after nine years that Dr. Tenma learns the perpetrator of the hospital murders is none other than the boy he saved years before, Johan Liebert. Plagued by guilt, he resolves to find Johan and end the life of this "monster" he feels responsible for creating. Despite his mission, Dr. Tenma is a humanitarian who genuinely cares about the lives of others. These acts of kindness make him very influential with the people he meets.

Dieter

Dieter is a young boy when found by Tenma to be under the care of a man named Hartmann. Upon meeting Dieter, Tenma notices that the young boy is covered in bruises. It turns out that Hartmann was trying to make Dieter into another Johan, a kind of leader that had not been produced from 511 Kinderheim before. Dieter later becomes more optimistic after being saved by Doctor Tenma from Hartmann's physical and mental abuse. He follows Doctor Tenma in his search for Johan, partially to prevent Tenma from becoming a murderer, but also because he is fond of Tenma. He later teams up with Nina, to give her moral support whenever her traumatic memories resurface as he was also subjected to similar abuse.

Wolfgang Grimmer

A freelance journalist who is researching Kinderheim 511; he is also soon drawn into the search for Johan, as he decides to help Tenma. (Source: Wikipedia)

Eva Heinemann

Eva Heinemann is Doctor Tenma's fiancée and the daughter of the Director of Eisler Memorial Hospital. She leaves Doctor Tenma after he is demoted by the Director for disobeying his orders. After Director Heinemann is murdered by Johan, she tries to return to Tenma, then recently promoted Chief of Surgery due to holes created in the staff by the murder. After being rejected by Doctor Tenma, she later becomes an extremely embittered alcoholic, a three time divorcee who uses the money from divorce settlements to finance her lavish lifestyle. After burning her house down in a drunken rage, she wanders throughout Germany and is also caught up in the investigation of the Monster. Although she does not personally suspect Dr. Tenma as having caused her father's death, one of the things she lives for is to see him suffer in prison in retribution for his rejection of her, as she had previously turned over evidence incriminating Dr. Tenma to Inspector Lunge. Her relationship with Tenma can be characterized as obsessive. During the series, she becomes a main target of Roberto. (Source: Wikipedia)

Heinrich Lunge

Inspector Lunge is a dedicated detective belonging to the BKA, which is like the FBI of Germany. When Dr. Tenma is accused of murder in Düsseldorf, Inspector Lunge is assigned to Tenma's case. Lunge moves his hand to "store" information from his cases. He has such a successful track record for solving his cases that he believes that he can't be wrong.

Martin Reest

Martin Reest works for Baby and has worked for him for three years. He says he'll do any job as long as it doesn't involve women. He went to prison for eight years, charged for killing his girlfriend and her lover. Peter Čapek calls Martin up for a job; to look after and protect Eva Heinemann as her bodyguard after she was being targeted by Roberto. Very reluctantly Martin accepts the job and spends time looking after Eva, during which he becomes closer to her.

Julius Reichwein

A psychologist who deals in counseling as well as assisting recovering alcoholics. His character bears a physical resemblance to the American actor Wilford Brimley and Shunsaku Ban from Astroboy. Naoki Urasawa has used the physical appearance of Brimley in the Pluto manga as well. First seen in Chapter 49.

Roberto

A big, burly man and one of the many people from the orphanage 511 Kinderheim that Johan controls. He admires Johan very much, and often acts as Johan's bodyguard and henchman. He is also a professional hitman with substantial proficiency. (Source: Wikipedia)

Johan Liebert

Johan Liebert is the namesake "monster" of the story and the mystery of his past is the focus of the plot. He has been called a monster, the next Adolf Hitler and even the devil himself. Johan Liebert was shot in the head at a young age but saved from death by Dr. Tenma. Because of this, he is very grateful to Dr. Tenma. He has spent portions of his life in different places under different aliases and possesses an extraordinary level of charisma and intelligence. He uses his gifts to cruelly manipulate and corrupt others, often with no apparent end other than to cause suffering and destruction. His original goal, as he stated when he was young, is to be the last one standing at the end of the world, although towards the end he decides upon a different fate. One of the themes of Monster is how individuals are capable of transforming into monsters; Johan often acts as both a direct and indirect catalyst for this transformation. Johan also shows strong attachment to his twin sister, Nina Fortner/Anna Liebert and refuses to leave her behind.