Lawrence and Mobile of Scott Pilgrim, both of whom the story teases by mentioning why they are just barely offscreen as a Running Gag. Until they both appear near the end of volume five. When they do, Scott initially confuses them for Gideon, as they all have dark hair and glasses.
Bob's sister (who is the mother of Croix and Hunter)
B.A.'s mother (although she does become The Voice occasionally)
Crutch's 'old lady', Casey Mae
Crowbar, Switch's partner-in-crime was this for years until he finally appeared on-panel in Hawg Waller's.
Brian's uncle (and former guardian)
Dave's father and brother.
Little Ego's therapist.
Mary Jane Watson was this until her first appearance in Amazing Spider-Man #42. Before that, she was sort of a Running Gag by the writers, someone who Aunt May and Mary Jane's own aunt were trying to have Peter meet, but never succeeding. The first time she appeared, her face was hidden. When she and Peter finally met, it came as quite a shock to Peter.
Empowered's mother. (They sometimes talk on the telephone, but we never hear what she says.)
Traditionally, this is true for the original members of the Yancy Street Gang, the ones that Ben Grimm knew when he was a member; they are never fully seen in the comics, only heard from the shadows, from the windows of buildings, or shown with hardhats or other headwear covering their faces. However, the younger "new generation" of the Gang was seen in full.
The "Enemy" in Sturmtruppen is never ever shown, except for one single strip (which is still pitch black).
The Joker was given this treatment in the New 52. After the events of Batman: Detective Comics #1, he apparently "retired", and the only thing left of him is his face after he had another psychotic criminal surgically remove it for some reason. He still manages to cause trouble for Gotham — in the wake of his disappearance, he gets in-universe Draco in Leather Pants treatment from a bunch of idiots who dress up like him to form angry mobs that accuse Batman of murdering the Joker. His face (which is currently being held by Gotham police) is also being treated as an object of worship by the Joker's crazier fans. The few times Joker appeared, it was either in a flashback or an impostor. He finally returns in Batman #13, where he retrieves his face. This leads to an event called Death of the Family, in which he goes after every member of the Bat-Family.
As of Batman #36 Joker has made his official return with a new, albeit tight to the point of looking Uncanny Valley, layer of skin on his face.
Boneville from Bone is a whole ghost town. Despite all the references the Bone cousins make to it, creator Jeff Smith has never actually depicted it, saying because Boneville isn't the focus of the story. Whatever Boneville looks like is up to the reader's imagination.
Kind of averted in the Spin-off novels "Quest for the Spark" which was written by Tom Sniegoski (Jeff Smith still illustrated it) starts off at the Boneville adventurer's guild. Yet true to the original story, it still doesn't describe anything of what Boneville actually looks like.
The Director General in Union Jack is never seen, even though she's being scapegoated for British government's lackluster response to the R.A.I.D. attack.
Transmetropolitan: Spider's book editor is never shown, never described, never gendered and only referred to as "The Whorehopper".
Father Time, a character from The Smurfs cartoon show, is this type of character in The Smurfs comic book adaptation of "The Smurflings." His workshop does appear in the comic book universe, but although mentioned, he himself does not appear in the story.
The Legend of Wonder Woman (2016): The Candy family has a huge impact on the story as their actions spur Etta into action and eventually give her a convenient excuse to escape to an active war zone, but they're never seen, only her little brother Mint is ever named, and none of them are ever even in the same state as our characters during the comic.
Star Trek: Early Voyages: The Tholians are never shown even though they are mentioned as being a major enemy of the Federation throughout the series. When they launch an attack on the Federation in the two-part story "The Fallen", their Chakuun shock troops do all the fighting.