Йоган Ліберт

Johan Liebert is the older twin son of the Lieberts, whose father Michael was an East German trade advisor who defected to the West. He is the titular "monster" and the main antagonist of the series. The mystery of his past is the focus of the plot. He is called a monster, the next Adolf Hitler and the devil himself.

Johan was shot in the head as a child when his parents were killed in their home, but saved from death by Dr. Tenma. Because of this, he regards Dr. Tenma almost as though he was his father. He claims to love his twin sister Anna, and has some loyalty to her. Johan has spent parts of his life in different places under different names. He possesses charisma and intelligence but is also cunning, manipulative and deceitful; while he is seemingly kind, compassionate and loving to children, he is actually cold, cruel and fundamentally incapable of empathy.

He uses his talents to manipulate and corrupt others, often with no apparent end than to cause suffering and destruction.

His goal, as he stated when he was young, is to kill everyone that knows of his existence, and be the last one standing at the end of the world. He identifies with other killers, discovering their secrets. 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. During his "perfect suicide" he was shot in the head by a bystander and seemingly died, but was later saved by Kenzo Tenma.