Mark the Mailman

Mark the Mailman is a major character on the show, being the first friend to appear in the missionima. He gives William an invitation to be on Who Will Win? which is a fictional reality show in the second episode of season one. Ever since, he becomes one of William's friends and a major character on the show.

Mark the Mailman gets married in Mark & Mary Marry and have twin kids, Mike and Martha, some time between said episode and Swapper.