Walter "Walt" Crane (John Mahon) is Martin's brother and uncle to Frasier and Niles. He appears only once, in the episode Beware of Greeks, in which he and Martin meet again, only to become separated again by the end of the episode.
There is a continuity error in the show, as Martin states in a Season 1 episode that he doesn't have a brother. In the episode Author, Author, when Frasier and Niles are having a fight, Martin tells them a story about a dispute he once had with a partner. His first words are "Now, I never had a brother..." The rest of the story that Martin tells the boys turns out to be a lie, which he reveals to Daphne after the boys make up and leave.