Bleach with Busou Renkin, there are many similarities: there are monsters that eat humans, and they are fought by secretive organizations where each member has their own special weapon. The hero's weapon starts as a large blade that can shoot energy, and is later upgraded to a smaller form. The highest form of monsters are humanoid, with the power to use the same weapons as the heroes. More importantly for this theory, the main cast includes an Action Girl and a Ditz. In Buso Renkin, though, the ditz is the hero's sister, and the Hero ends up with the Action Girl in the end. Therefore, Busou Renkin was created by Haruhi, who prefers Ichigo to be with Rukia instead of Orihime. The difference in attitude between Ichigo and Kazuki is because that is how she writes Ichigo in her fanfics. (She likes the goofier heroes, like Luffy.)
Papillion dresses and acts the way he does just to get attention.Let's look at the life of Koshaku Chonou: he describes himself as "the kind of person no one in class remembers, someone invisible." (No friends). His family threw him out when he became terminally ill and thus, of no use. So he becomes Papillion and what does he wear? Usually an iconic and noteworthy outfit that you are sure to remember, but sometimes just a pot, and the mankini from Zardos. He also becomes a bit of a ham with huge threats that he never acts on. His obsession with Kazuki can be explained by the fact that he is the only person to ever really notice him before all of that. In short, he wants attention. (Considering the last shot of him in the manga is him on top of Tokyo Tower with thousands screaming his name, I say it worked.)
The source of the name Papillon.It's not just because the name Chounou has associations with butterflies, as does Papillon in French. In the book/film Papillon, there's detail about prisoners storing contraband via Ass Shove. When you consider how character!Papillon frequently pulls things from Trouser Space, you can definitely see a connection.
Moonface was a prototype revised humanoid homunculus.As seen when he uses Satellite 30, it is clear that his mask is actually his face. The only other humanoid homunculi that have bodily modifications as extensive as that were the RHH.