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. Listen to how happy he sounds in Episode 4 that Tenma remembered him...
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.
Johan suddenly gets real emotional (as Anna) when Suk talks about how his mother didn't want him to become a cop. He actually seems concerned for Suk, telling him his mother was worried for him and asking him not to put himself in danger by investigating further. He may have just been saying what he thought his sister would have, but still...
Lunge's obsession with Tenma seems to have shades of this, at times. Especially when he's taken off his other cases, and he smiles at a photo of Tenma, telling him, "Now you're all I have left."