My theory always was that the Monk was a bit of a fanboy of the Master - the Peter Pettigrew to his Voldemort - who tried to emulate his success but was never quite as villainous until he was disconnected enough from him to become more dangerous (as I gather he was in the Big Finish audios). Whilst the Rani was quite determined to rise above all of them. Would be interesting if Chris revealed that the four characters were all part of the same Agency scheme.