There are three that so far that should have their own character guide.
In many versions of Charlie Brown, there is Miss Othmar, and numerous other adults that are voiced, on most occasions, by a musical instrument.
In Gofrette, there is Red, who is also voiced by musical instrument.
In Keeping Up Appearances, there is Sheridan who appears only in photographs and silhouette.
However, all of these characters have numerous conversations, in all or most episodes, with the starring characters.