On The Drew Carey Show, Drew's first boss, Mr. Bell, was never seen; only heard through Drew's speaker phone. (Although he was finally seen in the episode in which he was fired; he was shown carrying a box of his belongings out of his office.) His replacement, Nigel Wick (played by Craig Ferguson), was, of course, seen all the time.
On WKRP in Cincinnati, overnight jock Moss Steiger was mentioned, but never seen. Midday jock Rex Erhardt may also qualify, although he was finally shown once, in an episode in which he filled in for Dr. Johnny Fever. (He was played by Sam Anderson, one of many one-time roles Anderson played on WKRP.)
If we can count cartoons, Dr. Claw from Inspector Gadget qualifies as well. Only his right hand is shown and his voice is heard; his face was never shown.