Kabel was a timid, officious Lugal-Irra-Kush who had taken up residence on a trap street, hidden from human view in the centre of London to harbour alien refugees.
Kabel, along with his friend Rump, was one of the residents of the street responsible for greeting and processing new arrivals and assessing any alien claim for sanctuary on the street. It was Kabel's testimony of the night that Anah "died" that first alerted the Twelfth Doctor to another possible suspect. Rigsy had cried out for help from the Doctor on discovering Anah’s body, which must surely have alerted Ashildr to his connection with the Time Lord. However, the Mayor hadn’t mentioned it on their arrival.
Unknown to the Doctor and Clara, Rigsy's arrival on the street had been no accident and Anah's death was faked by Ashildr to lure the Doctor and allow him to be kidnapped. (TV: Face the Raven)