I havenīt played Dirge of Cerberus but I always thought Caith Sith was "living-robot" who only took orders from Reeve but when it talked it was Caith Sith talking not Reeve. Reeve was just constantly monitoring Caith Sith and let it now if something important happened.
Remember you can see Caith Sith in Honey bee Inn or at least hear his theme. What on earth was that cat doing there? Is Reeve getting some sort of pleasure when his robot gets expensive services?