"I realized that human lives have equal value by saving you! Nobody has the right to take another life! I’ve been living as a doctor with that engraved upon my heart for years!"
- Kenzo Tenma, to Johan Liebert
- Dr. Kenzo Tenma is the main protagonist of Monster and is the series' primary character.
- Tenma's early life is, to the greatest extent, unknown, save for one flashback. He is known to have been a sensitive child. During his youth in Japan, he was mocked and called "sissy-pants Tenma" by his peers, due to his habit of wetting himself and hiding in the surrounding forest. It would appear this sensitivity and anxiety carried on into his adult years in certain forms. Tenma certainly must have had a great respect for all life, despite his treatment, in his youth for it to have carried on so profoundly during his adulthood. Thus, what can be assumed about Tenma during his childhood is that he was a sensitive individual who held life in very high esteem, and that this would carry into his adult years later on.
- Starting out as a prominent Japanese neurosurgeon at Eisler Memorial Hospital in Dusseldorf, West Germany, Tenma works for Director Heinemann and is engaged to his daughter, Eva, as of 1986. However, he shows growing contempt for the hospital administration's lack of regard towards patients of a lower class status and of "less importance." The series begins with Tenma treating the famous opera singer F. Rosenbach at the expense of a Turkish patient's life. The Turkish man's wife becomes upset with Tenma, angrily and with tears streaming down her face, asking him why he didn't save her husband. As she mercilessly but ineffectually pounds her clenched fists into his chest, Tenma himself begans to realize the gravity of the situation and what he could've done to save the man were it not for the hospital's policies. He confides in his friend and colleague Dr. Becker, who, while symphathizing, doesn't help in calming Dr. Tenma's growing feelings towards the hospital beaureaucracy. While at a dinner engagement with Eva later on, pondering on the situation with her, he becomes visibly disgusted when Eva claims that not all life is equal.
- Tenma's contempt for this system finally manifests itself when he saves the life of a young boy who is taken into the hospital with a gunshot wound in his head, rather than saving the life of Mayor Roedecker of Dusseldorf, as he was told to by Director Heinemann and Dr. Oppenheim. The boy, one of two twins adopted by the Lieberts, who recently defected from East Germany, is in critical condition, but Tenma nonetheless manages to save him. However, when the Mayor dies, Tenma's actions cost him his position and Eva, and Dr. Boyer is placed in charge of the twins. However, when Director Heinemann, Dr. Boyer and Dr. Oppenheim die after eating mysteriously poisoned candies, Tenma makes his way back up through the ranks of the hospital, becoming Chief of Neurosurgery once more. At the Director's funeral, Tenma meets Inspectors Egon Weisbach, who has been placed in charge of the Liebert case, and Heinrich Lunge of the BKA, the latter of whom immediately becomes suspicious of Tenma in regards to the Director's and his colleagues' murder. Later, Eva attempts to reconcile with Tenma, but she is turned down and Tenma walks away from their relationship.
- Ten years later, in 1995, a criminal named Adolf Junkers, who has been hit by a car while attempting escape from someone and who continuously refers to the existence of a "monster", is taken to Eisler Memorial Hospital under Inspector Lunge's supervision. After Tenma operates on Junkers, he befriends the man and they become closer as he takes care of Junkers. However, one night, Junkers has gone missing, and Tenma finds the security guard posted on duty next to his room dead from another poisoned candy. Pursuing Junkers' trail to an abandoned building, Tenma finds Junkers, frightened and kneeling before a shadowy figure, urging Tenma not to come any closer. Junkers goes on to explain that this was the "monster" he'd been talking about earlier, and that this man had killed all of Junkers' partners before targeting him. As Tenma gets closer and the figure begins to speak, he realizes that it's, in fact, Johan Liebert, the boy he saved so many years before. Pleading with Johan not to shoot the man, Tenma realizes with horror that Johan is, in fact, a serial killer, and that he murdered the Director and his colleagues by poisoning the candy when Tenma angrily said that they deserved to be dead after being deprived of his position, thinking Johan was still unconcious when he said it. Junkers, suddenly deciding he wishes to live after all, begs Tenma into saving him, is shot by Johan three times and dies. As Johan walks away from the scene of the crime, he thanks Tenma once more for saving him and hints that he considers Tenma to be a father to him as a paralyzed Tenma listens, before leaving. Tenma is then brought in for questioning about the incident and Inspector Lunge grows ever more obsessed with the idea that Tenma is a murderer. Afterwards, Tenma resolves to hunt down Johan, and undo the mistake that he created in bringing Johan back to life.