Kevin works as Cannon Black's gardener, a job that enables him to spy on the Canon and find out vital information as he peeps through the windows, pruning the hedges.
Because Kevin has Down's Syndrome, the Cannon doesn't even conceive he could possibly pass on information, making Kevin both invisible and invaluable.
Like a blind person who has developed an enhanced sense of hearing, Kevin can sense the presence of demons and is often an early warning that something devilish is about to happen in the vicinity.
Timmy Lang played Russell in an episode of Peak Practice called Walls of Jericho and Billy Conrad in A Touch of Frost, in the episode Appropriate Adults.