I bet this is going to sound kinda ridiculous, but after reading a few fanfics about Killer a thought started to form. Basically my thought process was that there was a possibility that Rosinante was Killers dad, I mean there are some similarities in both of their designs. But unfortunately that can't be proven true... Wouldn't it make sense that Killer wore a mask to hid his face, because 1) he wanted to hid his laugh from others. And 2) so that no one could see the slight resemblance between him and Rosincantes.
Unfortunately due to my own curiosity, I discovered it was just a very off base theory of my own. Why? Its mainly due to the fact that Killer was born about 12 years after Rosinante. But for some that doesn't stop me from thinking that they are somehow related... Which with further investigation is starting to look completely impossible...