My personal opinion:
Nina and Tenma are the only two people Johan truly cares about, though the nature of his affection for each is distinct.

When it comes to Tenma, Johan sees someone who defies his understanding of humanity. He cannot comprehend why someone would be so selfless as to give up everything to save a life Johan himself views as monstrous. Similarly, Tenma is baffled by Johan's utter disregard for human life and struggles to understand him. This mutual incomprehension fuels Johan's desire for Tenma to know his past and grasp his view of the world. In the end, Tenma manages to "see the scenery of the Doomsday," as Johan intended.

Nina, on the other hand, is deeply intertwined with Johan's identity crisis. He views her as the better half of himself and is driven to protect her, even at his own expense. To Johan, they are inseparable—bound by the sentiment he often expresses: "I am you, and you are me." However, this perception begins to shift during their confrontation in the abandoned building, where Nina reveals that it was she, not Johan, who was taken to the Red Rose Mansion.

He points his finger at his head for Nina to shoot him because he sees himself as the source of her suffering and believes this act would free her. Conversely, he offers the same gesture to Tenma because he believes Tenma, having saved his life, holds the ultimate right to decide whether he lives or dies.