Johan Liebert is the titular "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 regards Dr. Tenma almost as though he was his father. 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 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.

Spoiler

During his "perfect suicide" he was shot in the head by a bystander and seemingly died, but was later saved by Kenzo Tenma