Oh man, fuck me, what did I get myself into with 20-ish endings?! XD (I might've added another Sev death ending... oops, my bad.)
As for your question, Rosario's snake-like traits mostly come from his artistic design/character sheet— I just really wanted a character with a slit tongue (makes the kissing scene extra spicy). Then I figured, why not take it up a notch, especially since he can shapeshift~ It also ties in with the medicine thematics, and, well... snake oil solutions that he offers when he's really just up to his own tricks. Oh, and not gonna lie, I also really wanted to throw in a hemipenis LOL