Doctor Kenzo 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.

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.

"Liebheart" is Johan's official last name according to the Japanese anime page, but "Liebert" is his official name according to the English translation of the manga. Please leave his name as-is.