Johan Liebert is the titular "monster" of the story who serves as the principal antagonist and the mystery of his past is the focus of the plot. He has been called a monster, the next 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 regards Dr. Tenma almost as though he was his father. While he claims to have a deep love for his twin sister and is shown to have a loyalty to her, he seems undisturbed by the fact that his own henchman Roberto was ready to have her killed to keep her from him. He has spent portions of his life in different places under different aliases and possesses an extraordinary level of charisma and intelligence. Along with that incredible intelligence, comes with his cunning, manipulative and above all excellent with deceitfulness. As he is he was once seen able to convince Detective Richard Braun that he was lying about being an alcoholic. While he is shown to be kind, compassionate, and loving to children; it should be noted that he can be cold and very cruel. He even seems to have a good idea of how minds will work, an example would be when he sent a child witness that had seen him (dressed as Nina) into a red light district. Its most likely he knew that the boy would be devastated by the perversions and cruelty in the area. He uses his gifts to cruelly manipulate and corrupt others, often with no apparent end other than to cause suffering and destruction. His goal, as he stated when he was young, is to be the last one standing at the end of the world.
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. He also seems to identify with other killers and finds out about things that they, themselves never told anyone. Like Tenma, Johan also has many similarities to a character from a classic manga by Osamu Tezuka, in his case Michio Yuki, the main villain from MW. These include his childhood involvement in a secret military experiment, ability to skillfully manipulate powerful people, ambitions to cause the end of the world, occasional suicidal tendencies and his infrequent bouts of cross-dressing. In keeping with the theme of the "Apocalypse", Johan shares many similar traits with the Antichrist (being "resurrected" after being pierced through the head, being mistaken for a monster with "seven heads and many horns"). Although it is seen in Nina's memories that Johan and Anna had no real names, he seems to mostly go by the name Johan. After doing acts that ended up causing the demise of Petr Capek, he orchestrates the Ruhenheim Massacre and has his final encounter with Dr. Tenma. When Johan threatens Wim in order to get Tenma to shoot him, Johan ends up shot by the boy's drunken father Herbert and his body was found by the ambulance with the bullet wound treated by Dr. Tenma. His comatose body was later placed in a police hospital when Tenma visits him. The final scene shows his bedroom empty with the window opened indicating that Johan recovered and escaped.