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.

Shigeo Kageyama, nicknamed "Mob" (モブ), is the protagonist of the Mob Psycho 100 series, as well as assistant and disciple of Arataka Reigen. He is the newest member of the Body Improvement Club. As a child, Mob learned that his psychic power was linked to his emotions. Strong negative emotions can cause him to hurt others, something he strongly dislikes and avoids. Therefore, he generally tries to keep his emotions in check. He is rather socially awkward. He is incapable of "reading the atmosphere," which has lead to him having a lack of friends. Mob is very compassionate. He is forgiving, kind and helpful. He tries to be kind to others in many instances. He is morally opposed towards using psychic ability against other people. Another principle of his is that psychic ability does not make him or others inherently better and that his psychic ability is not something that he requires in life. This belief leads to conflict. (Source: Mob Psycho 100 Wikia)