Who is 'Dirty Harry', the ISI's second-in-command that former Pak PM Imran Khan 'fears'?

India Today has pulled out a letter from slain journalist Arshad Sharif’s mother to trace the ‘brigadier’ or ‘Dirty Harry’, whom Imran Khan mentioned in a video, hours before being arrested. Meanwhile PTI supporters share videos claiming to have set this top Pak army major gen's house. Who is 'dirty Harry' AKA Faisal Naseer that Imran Khan has publicly accused of conspiring to assassinate him?