Due to Ritsuka Fujimaru's consciousness being sent into that of Juliet's friend, Ritsuka is unable to comprehend all of the sensory data in regards to humans through that connection. Ritsuka views them as Servants contracted to them, Cain appearing as Mephistopheles. The assigned Servants may have some connection in terms of personality, but it is possible for key traits of the person to be confused due to the process.
Cain doesn't like the business of the crime family, so he has pretended to be feeble minded most of his life to avoid being dragged into it. He presents a mental age similar to that of the young Lorie Goldie.