Roberto's worship of Johan in Monster drives past Battle Butler loyalty and straight into some major Ho Yay when he, under the pretense of a new patient, starts telling Dr. Reichwein about his "affair":
"Then I met him... A beautiful young man with blond hair. No matter how many I've killed, no matter that I've lost my country, It was all like a dream. But...it's been different since I met him. Everything seems real...I'm not lonely anymore."
Also, Johan and Tenma. "My Dear/Favourite Doctor Tenma", indeed.
Johan is basically a walking HoYay generator. Throughout Monster we meet several different men with fanatical emotional attachments to him, whom Johan Mind Raped into killing people for him and then either killed or drove to suicide. These men tend to describe Johan as "beautiful" and as having given their life meaning. It's never explicitly stated how Johan gets their undying loyalty, but what he does to Jan Suk in canon proves that he's nothing if not resourceful.