 Niles wife Maris is never seen (at least her face) or heard from in  the entire series. The same thing is mentioned about Vera, the wife  of Norm Petersen in _"Cheers" (1982)_ (qv). We only hear of Maris  from Niles about what is wrong about her. This method is used again  when Martin meets the girl he has been watching from across the  street via his telescope.