As a rule of thumb, The Emperor of Japan (both the living, historical, or even fictitious ones) is never shown or even mentioned in any Japanese media other than news stories or story books. See this  for details. On the other hand, they don't have any qualms to show any daimyo, shogun or even Prime Ministers (both historical and fictitious) in any kind of light. Some clever writers manage to include an equivalent of the Japanese Imperial Family through some good writing:
Subverted in Samurai Pizza Cats at least between the Japanese and American versions: Little Tokyo is ruled by a brainless emperor in the American iteration, but the character is described as a Shogun in the original Japanese airings.
Averted in Oda Nobuna no Yabou with Himiko, who is also a girl by making her a Captain Ersatz: She is "the head of Yamato court" and said to be an incarnation of the Gods who built Japan, something any Japanese would immediately associated with the empress—but she was never referred to as the empress of Japan. She's also notable to be the only character of the series when the number or fan-art (erotic or otherwise) about her can be counted with a single hand.
Averted in the Harukanaru Toki No Naka De manga for the original game. The Emperor of the parallel Heian Japan-like universe face is seen when he speaks with his brother Eisen - we even see him give Eisen advice on how to interact with Akane.
Toji's sister is mentioned at various points in Neon Genesis Evangelion, and is important enough that she serves at the main reason he becomes a pilot. Despite this, she never actually appears in the series at all. She does however have a very brief non-speaking cameo in the second Rebuild of Evangelion movie. She then becomes an Ascended Extra in the third movie, and is finally given an actual name ("Sakura").
This is true of several other characters. For instance, Hikari mentions several times that she has two sisters, but we never see either of them.
The Dancing Giant ghost in Haunted Junction is only seen from the knee down, and generally only one leg at a time. He's that big.
Ai Enma's "grandma" from Hell Girl. Although she talks, all we ever see of her form is her silhouette. The only human that takes a good look at her is completely horrified by what she saw.
Partial example: Tsunade from Narutoalways makes herself look much younger than she is, and on the couple of occasions where she was exhausted enough that the illusion fades we're prevented from getting a good look at her. What little we do see implies that she actually looks older than she really is, no doubt due to the life-shortening effect of her ultimate healing technique.
In Taro Kid/Skyers 5 (name varies depending on jurisdiction) the Big Bad (head of a SPECTRE-like criminal organization) is only ever seen from behind his chair.
The Sacred Ancestor, the Vampire King Dracula remains unseen save for flashbacks where he receives no physical description save for his eyes in all mediums of Vampire Hunter D.
In Sakura-sou no Pet na Kanojo, Ryunosuke, resident in room 102 of Sakura Hall is one of these. Early on, his only interactions with the other characters are over the computer, either via chat room dialogue, or more commonly, a maid AI program that does the speaking for what he's thinking about. He finally appears onscreen in episode 9, and is also visible in the credits.
Quite a few of the bigger players in FLCL are never shown, including Medical Mechanica, Haruko's higher-ups, and Naota's older brother (even when there's a picture of him, his face is never shown.)
In The Vision of Escaflowne Hitomi occasionally referred to her younger brother, we never see him once at her home in the present day.
Ben Grimm often speaks fondly of his dear Aunt Petunia, but for decades she was never seen. Finally, Fantastic Four #239 featured her in person. It's revealed that she is a young woman married to Ben's Uncle Jake. Another couple decades later, she is Stuffed In The Fridge.
In the early Spider-Man comics, Mary Jane was only referred to and never fully seen, although one story did featured a brief cameo we could only see her body and not her face. It would be over two years before readers received their first full glimpse of her in the comics.
In Monsters In Paradise, Hello-san, the story's Big Bad, is this. When Koishi meets him for the first time, she finds it impossible to discern his appearance as she cannot see him at all.
In the 1980 Claymation educational film Dinosaurs!, neither the main character Phillip nor his classmates are ever actually shown, and his teacher is only briefly seen from behind. When this film was released to home video in 1987, new scenes were added with Phillip being played by Fred Savage. The new footage featured Phillip's unseen mother, as well as a mysterious female voice who educates him on dinosaurs.
The supposed god in 10,000 B.C. makes sure no ones sees him to add to his whole mystique. He even had all blind servants.
Not only not seen, but not heard either. Sellers does a Newhart-style telephone gag where, though you only hear President Merkin's side of the conversation, it's pretty clear what "Dmitri" is saying.
Merkin: I'm sorry too, Dmitri. I'm very sorry. ... All right, you're sorrier than I am. But I am sorry as well. ... I am as sorry as you are, Dmitri. Don't say that you're more sorry than I am because I am capable of being just as sorry as you are. So we're both sorry, all right? ... All right.
Roxy Carmichael from the 1990 movie Welcome Home Roxy Carmichael.
Mainly because he's in Russia, which is only shown in the movie as a background-shot.
But he does appear very briefly, in the illustration on his page of the storybook. From the looks of things, he must be the youngest knight of them all...
The picture is actually of Michael Palin's son.
The man directing the criminal organization in The Testament of Dr. Mabuse is only shown as a silhouette behind a curtain giving orders. It's later revealed that the man behind the curtain is just a cardboard cutout and a radio; the head of the psych hospital, who has become obsessed with patient Mabuse's titular "crime manual," has been remotely directing the organization the whole time.
The title character in Edward, My Son is never seen onscreen. Everything we know about him we hear from other people.
Much like Dr. Claw Mr. Feather's boss "The Man" from Undercover Brother only the back of his head and his hands are seen.
The 1976 biopic of the Prophet Mohammed, Mohammad Messenger Of God (US title The Message ), complies with Islamic law by never showing the Prophet or any of his immediate family. This leads to a lot of very meaningful shots of his camel and his camel-goad.
Technically, this tactic was used before in the 1959 Charlton Heston film Ben Hur. Jesus cameos in the film, giving water to Judah Ben Hur and, later, when Ben Hur witnesses the crucifixion. Not once is Jesus' face shown. Many Christians, especially Roman Catholics, are uncomfortable with any depictions of Jesus' face in contemporary or even classic media, as it puts a potentially misleading visual "face" on Jesus. The director of Ben Hur decided not to show Jesus' face in the film out of respect for this.
Pippin, Aragorn, and Denethor see him face-to-face through the Palantír. Denethor explains Sauron's hidden nature as being due to the notion that all wise persons of power use others to do their fighting for them; however the story indicates that this is the act of dictators (like Sauron, Denethor, Saurman etc), while good rulers fight actually lead their underlings into battle (like Gandalf, Aragorn, and Théoden did).
Denethor isn't being cowardly, though. Before he goes insane from the use of Palantír he essentially tells that if everything is lost and Gondor falls, he will be spared so that Sauron can personally gloat and mock him, and then he will have a chance to strike personally, revealing that he's been living and even sleeping in a chain mail with sword on his belt to prepare for that moment. That is the reason he won't endanger himself in lesser battles. "Historically," Denethor is recorded by Tolkien as personally leading many defenses of Osgiliath, and every bit Boromir's equal in the pure badass department.
Somewhat undermined in the movies where the Eye of Sauron shows up plenty of times, such that the audience assumes (erroneously) that Sauron is the Eye.
In fact, in the books it's made pretty clear (though on few occasions, readers may miss it) that Sauron has a physical body (though he's still missing the finger Isildur cut off). Gollum was tortured by Sauron personally, and Aragorn's challenge for him to "come forth" at the Black Gate is meant literally - Aragorn wanted to personally fight Sauron, and so presumably knew or at least thought he had a body. In The Silmarillion, we're told that The Eye existed before Sauron lost his body.
The third film originally had a scene where a ghost-like Sauron would have confronted Aragorn face to face. It was cut so as not to distract from the events taking place on Mount Doom.
If you look at the events of the Silmarillion, Sauron's refusal to fight is well-founded. Every time he has involved himself personally, (against Beren and Lúthien; against the Númenóreans; against Isildur at the end of the Second Age) he has been defeated. Does this make him a coward, or just the result of his "being evil"? You decide.
It's implied that Sauron keeps himself secret even from his most high-ranking servants. Some have suggested that only the Mouth of Sauron (the only one of Sauron's servants allowed to refer to his master by name) ever actually talks to him.
Galbatorix, the Big Bad of the Inheritance Cycle, has yet to put in a physical appearance in the books. He does appear in The Film, played by John Malkovich, but this was only for appearances' sake. He finally shows up in the last book.
By extension, his dragon Shruikan isn't seen until they confront Galbatorix in his throne room.
Altered Carbon includes the often-referenced but never-present Elias Ryker. It emerges that the central character, Takeshi Kovacs, is inhabiting Ryker's body or "sleeve" while Ryker himself is "on stack," imprisoned in a digital environment.
The Soldier in White from Catch-22. A hospital patient so covered in bandages that no one knows anything about him. No one even realizes when he dies, until a nurse takes his temperature and he doesn't have one. One soldier speculates there's nothing under the bandages, he's just a hollow plaster shell.
Also, the Dead Man in Yossarian's Tent (Lt. Mudd). He died on a mission before officially checking in at the base, therefore the army brass refuse to admit he ever existed, causing a paperwork nightmare.
Live Action TV
''Monty Python's Flying Circus'! had the episode 'Michael Ellis' which involved Eric Idle being repeatedly mistaken for the mysterious Mr. Ellis, who is never seen.
From The Big Bang Theory, the mother of Howard Wolowitz. And given her shrilling voice and what we know about her, it sure as hell better stay that way.
In "The Countdown Reflection" she can bee seen from a birds eye view thanks to the Google Earth satellite from Leonard and Sheldon's apartment roof.
In "The Spoiler Alert Segmentation" She is seen through a slightly ajar door 3 times each lasting around a second or two each only her back side is seen as well as the back of her brunette hair.
His hand is seen in one episode, grabbing at Karen's breast.
In Scarecrow and Mrs. King, there was a character in the first season or two (later phased out) who drove around in a limo giving out orders/assignments. The name of the character was probably "Mr. Blue."
On Sports Night, Luther Sachs was the owner of Continental Corp, which owned Continental Sports Channel, which broadcast the titular Show Within a Show. Though there were many times he was referenced (through such things as people having to take a call from him or having to go meet him, etc.) he was never seen or heard.
In public-relations specials, these characters need not even be fictional. After playing The Voice (Carlton the doorman on Rhoda), Lorenzo Music (who also provided the voice for Garfield) later appeared on a Garfield TV special with his back to the camera at all times, as Garfield's creator Jim Davis remarked that he'd never seen his face. The producers of an E! special on Beanie Babies did something similar with the founder of the Ty corporation.
Bill Wilson, founder of Alcoholics Anonymous, and Gravitys Rainbow author Thomas Pynchon, both of whom refuse to appear on camera throughout their whole careers.
In his first interview after leaving the band in 1980, former KISS drummer Peter Criss appeared with his back to the camera, apparently at the insistence of his former band mates. At the time, KISS had never appeared in public without their trademark stage makeup. Criss would reveal his face in a television appearance later that year. The band would abandon its makeup in 1983 in an "unmasking" on MTV.
Similar to the first example, VH-1's I Love The 90s had a segment on MovieFone, with the celebrities commenting that they had no idea what "Mr. MovieFone" looked like. They actually had the actor, Russ Leatherman, on the show.
Linderman of Heroes was this for much of the show's run, often referenced with characters seen speaking to him over the phone, or middle men conveying his orders. This makes sense as he's very much the shadowy manipulator but it was actually because the show couldn't afford to have a big star like Malcolm McDowell appear in too many episodes.
This makes it even more strange to see a kind, grandfatherly character as the Big Bad, while we have been assured that "Mr. Linderman" is a truly ruthless, merciless villain. Informed Ability anyone?
Except that he really is an evil manipulator, so he's closer to Affably Evil.
Moonlighting characters often referred to the "Ensalmo Case," which sort of combines this with Noodle Incident: The case was only mentioned in passing. There were no characters named Ensalmo, nor did the case ever come to a close. It's revealed in the Grand Finale that it was never solved.
And, of course, the ultimate example, Charlie, from Charlie's Angels. He usually appears as The Voice, but has sometimes appeared as The Faceless in several episodes and only finally reveals himself to the Angels during the final episode, "Let Our Angel Live" when he shows up at the hospital to be by Kelly's side after she is shot.
Chef in Star Trek: Enterprise. In the last episode, he's revealed to be... William Riker? Actually, Riker was on the holodeck playing Chef.
Captain Boday in Star Trek: Deep Space Nine. He was said to have various strange features, such as a brain visible through his transparent cranium.
The banker on game show Deal or No Deal. His body can be seen from the booth, but there is no lighting on him, making him practically a shadow.
In the UK version, he is simply the voice on the other end of the telephone.
Norm's wife Vera in Cheers, although she was occasionally given a voice or partially shown.
Also Sam's brother Derrick, although he was given a voice.
Maris of Frasier. The creators had initially planned to show her, but the descriptions of her made her so monstrous that after a few seasons, they couldn't think of a way to show her without underselling the character.
Also the source of a hilarious subplot involving Niles getting a pet whippet that everyone agrees is basically a canine Maris — this is basically the closest the audience ever gets to seeing the woman.
Except that we did see a heavily bandaged-up Maris in a flashback in one episode set in a hospital, but it was brief and she was almost entirely covered up. Another episode showed us her silhouette against a shower curtain.
Arthur Daley's wife in Minder, fearfully referred to as "'Er Indoors."
Mrs. Columbo. Some fans theorize she doesn't exist, partly because some scenes really stretch the feasibility of it (she's nowhere to be seen at the policeman's ball despite Lt. Columbo asserting she'd be there), and partly because it ties into theories abouthow Columbosolves his cases.
Chuckles the Clown was one of these for the most part, although he did appear onscreen in a couple different episodes (played by a couple different actors).
As noted, Carlton the doorman in the spin off, Rhoda.
Don Pardo announced the prizes on many game shows, and the cast on Saturday Night Live during the '70s, without ever appearing on-camera. One SNL skit lampshaded this by having Pardo appear as, literally, an invisible man; auditioning for the SNL announcer gig ("Don Pardo? Is he still alive?"), his invisible presence sits down in a swivel chair which visibly moves and audibly creaks.
Subverted on the NBC episodes of The Price Is Right, as he would occasionally be seen on camera as a substitute host whenever regular host Bill Cullen was either sick or on vacation.
On LOST, the "monster" was unseen for the whole first season.
Subverted: We saw the Monster repeatedly throughout the first season, except we didn't know it was the Monster because we didn't know it had the ability to do a Dead Person Impersonation. However, we didn't see its actual Smoke Monster form until season 1 finale, and we never saw its real form until the season 5 finale.
Seinfeld had several, most notably George's Boss George Steinbrenner, whose face is never seen, Kramer's never-seen friends Bob Sacamano and Lomez, and Jerry's cousin Jeffrey, about whom Uncle Leo talks constantly.
George Steinbrenner was then real part owner and managing partner of the Yankees when George supposedly worked for the team. Steinbrenner was in the news a lot, and everyone knew what he looked like. Rather than try and get the real Steinbrenner to play himself, or find an actor who looked uncannily like him, whom audiences would accept, the producers chose to use someone who just looked like Steinbrenner from the back.
In Twin Peaks, Agent Cooper constantly recorded messages to "Diane" with his mini recorder, but Diane herself is never seen or heard. She does apparently mail Coop some earplugs at one point, however, so she is assumed to actually exist.
In The X-Files, Mulder often made calls to someone at the FBI named "Danny," requesting warrants, paperwork, or other small but difficult-to-retrieve items they needed in the field. Sometimes Scully called Danny, and once John Dogget did as well. However, audiences never got to see Danny. The closest Chris Carter ever got to a character description was once during an interview when he joked that Danny was a gnome that lived in Mulder's desk.
The extent to which Burnside is/is not willing to reconcile with his ex-wife Belinda in The Sandbaggers is a plot point in several episodes. However, she is never seen.
Dan & Becs is absolutely made of this trope. Although the only characters to appear onscreen are the titular ones recording their video diaries, between them they reference dozens of characters. Just for starters: their respective parents, Becs's sister, their various respective friends, their various respective friends' siblings, Becs's agent, Dan's various exes, the various people they meet when looking for work... the list goes on. And that's not even starting on the real-life people who are unseen characters within the show.
On The Andy Griffith Show, Goober started out as one of these; his cousin Gomer would tell Andy, "Goober says 'hey,'" when the two men met. Goober finally made an appearance (alongside Gomer) in the season 4 episode "Fun Girls," and after Gomer left for his own show Goober essentially became his Suspiciously Similar Substitute.
Sarah, the town telephone operator.
Then, of course, there was "Juanita," the waitress that Barney sometimes flirted with on the telephone.
Mash: Sparky, the radio operator at I-Corps who Radar or Klinger often speaks with, pretty much fits this trope... although he did make a single, brief appearance in the season 1 episode "Tuttle."
The camp's P.A. announcer was both this and The Voice.
The characters' loved ones back in the States, obviously...although some of them did "appear" via home movie, including Henry's wife Larraine, Frank's wife Louise, B.J.'s wife Peg, and Radar's mother (played by Gary Burghoff in drag).
B.J.'s wife Peg appears again in the surreal episode "Dreams", in a depressing Nightmare Sequence that is meant to show how much he misses her and how he believes the war has pulled him away from her. Naturally, everyone else is suffering a case of this throughout this particular episode.
In the "Run for the Money" episode, Charles gets a tape recording from his sister Honoria, so we get to hear her voice (which includes a bad stutter).
Colonel Potter's wife in the states, Mildred, does not appear in the main series (save as a photograph on his desk), but is a regular character (played by two different actresses) in the short-lived spin-off AfterMASH.
The physical incarnation of this particular trope is undoubtedly Wilson from Home Improvement. If it wasn't the fence hiding his mouth, it was a tree branch, a grill, other people, a Santa beard, or even a yam!
Nils' mother Elna in the first season of Norwegian sitcom Mot I Brøstet, only referred to and heard through one-sided phone calls for the first 7 episodes. The sequel series Karl&Co did the same with Ulf's wife Magda, who stayed unrevealed throughout the entire series.
Captain Mainwaring's wife Elizabeth from Dads Army, who is apparently much scarier than the Nazis. We get the briefest of glimpses in one episode where the two are lying in bunks, with the captain on the bottom. Above him in the top bunk is an extraordinarily massive indentation, suggesting a literal mountain of a woman.
Peggy's mother in Married... with Children. She is implied to be amazingly obese, so perhaps the idea is that the camera couldn't have fit her in?
In Keeping Up Appearances, Violet was this for the earlier series. She eventually made her full-time appearance as a character later in the run.
Sheridan was this for the whole series.
Amber, daughter of Tim and Daisy's landlady Marsha in Spaced. The others hear her having raging (but indistinct) rows with her mother, and at one point she storms downs the stairs played by the producer's sister in a wig, but neither they nor we ever meet her or see her face.
Robin Masters, the author who owns the estate on Magnum, P.I.
Kim's "best friend" Tina on Kath and Kim never seems to spend any time with Kim at all.
Angela Valentine was a girl in Beaver's class at Grant Avenue Elementary School in Leave It to Beaver. She was mentioned in no fewer than 16 episodes throughout the series' run. Usually concerning her sixth toe or getting sick in class, having a real gold filling, the eating of library paste, winning a tool chest at the local theater, suggesting (and then winning) a class beauty contest, losing her bite plate on the playground, bringing a "baked Alaskan" to the cake sale, getting her extra toe cut off, calling Beaver a "dirty, rotten, smelly old apple," having a birthday party, getting sick in the back of the school bus and wearing a Jackie Kennedy wig to school. (She is shown very briefly in 2 episodes but only from behind and later running or walking by. No one seems to know who played the part.)
Also, Larry Mondello's father.
Martha's often talked about boyfriend Chet is never seen on screen. And since he was killed off in episode 3x02, it's doubtful that he ever will be.
Friends has "Ugly Naked Guy" who is blessedly always off-camera.
The only time "Ugly Naked Guy" appeared on-screen, he was shot from the back (and thankfully above the waist).
First boss Mr. Bell in The Drew Carey Show. His voice is frequently heard over intercom, but he doesn't appear on-screen until the end of the first season (in which he's been fired).
The documentary film crews from all the various versions of The Office, who never speak or overtly involve themselves with the office workers, but who must have some offscreen relationship with them, especially in the long-lasting American version.
Red's wife Berince on The Red Green Show was never seen (except in a Christmas special near the series end).
Dalton's wife Ann Marie also went unseen for most of the show, but was later upgraded to The Voice.
Actually, the show was full of these people. There were a number of lodge members who were alwayso talked about but never seen, mainly Moose Thompson and Old Man Segewick.
For that matter there were often entire episodes in which unseen characters would have significant roles.
The Royle Family, which rarely leaves the confines of one house, creates a whole community of characters only ever described by the core cast. Chiefly; Jim and Barbara's neighbour 'Leggings' Lorraine, the housebound Elsie (who lives in the same flats as 'Nana' Norma and whose eventual death provides the background to an episode in the third series) and Dave's best friends Tony Macca and Gary. Most memorable though is Tony's supposedly tarty - and well-endowed - younger sister, Beverley.
Some characters are talked about in several episodes before we do eventually get a glimpse of them, including Anthony's best friend Darren and girlfriend Emma (plus her parents). Dave's parents were also regularly mentioned but didn't appear onscreen until the 2008 Christmas special. In addition, crucial aspects of Norma's character are introduced in the very first episode, when she makes a phone call to the Royle household, but it's not until later in the series that we see and hear her.
Adele, Sherrif Metzger's wife in Murder, She Wrote is often mentioned but never seen.
Kimmy Gibbler's parents from Full House is regularly mentioned by Kimmy, but is never shown onscreen.
Felix’s ex-wife Gloria did not appear until the second season of The Odd Couple.
Oscar’s girlfriend “Crazy Rhoda Zimmerman” was never seen.
John Bracken, studio head and title character of Brackens World, was not seen until season two.
In 2 Broke Girls, Caroline's father, Martin, was originally unseen — Justified because he's in jail, but both she and Max were shown talking to him on her phone. He has later appeared on screen, played by Steven Weber.
Until the last season Morty Fine, Fran's father, from The Nanny.
When the One Away game is played on The Price Is Right, Drew has the contestant plead to the "Mighty Sound Effects Lady" to see if they got X numbers right on the board. Naturally, said sound effects lady is never seen or heard on camera.
On The Golden Girls, Dorothy's crossdressing brother Phil is frequently mentioned and joked about but never seen. Rose's husband Charlie is the only former husband of the girls never seen; Sophia and Blanche's husbands are seen in flashback, and Dorothy's ex-husband Stan is a recurring character.
Queen Elizabeth II makes fairly frequent non-appearances on Doctor Who. It's partly a running joke, partly mandatory— evidently there are legal implications to using the reigning monarch's likeness on UK TV.
Not really. They probably just don't want to have someone playing her onscreen and it obviously not being her. Hence why in Silver Nemesis she's played by a lookalike shot from far away. (They'd originally wanted Prince Edward to play himself, but he graciously declined.)
Although she made one appearance in an episode of Frasier, in a dream sequence where Daphne is horrified to see her on the balcony enjoying a barbecue with Marty. She is played by an unconvincing impersonator, mind you.
If the titular mother in How I Met Your Mother does make appearances, we never see her face. In "Girls vs. Suits", Ted sees her ankle. In "No Pressure", only the back of her covered head is shown. In "No Tomorrow", the yellow umbrella is covering half of her body and she is wearing jeans.
Until the Season 8 Finale, when we finally get to see her face.
On The Sopranos, Janice's estranged son Harpo is mentioned repeatedly over the course of the show, but never seen.
The '50s sitcom December Bride had as one of its supporting characters next-door neighbor Pete Porter (Harry Morgan), who often spoke of his unseen wife Gladys. Morgan was popular enough to get his own After Show called Pete and Gladys...in which the latter did appear.
Samson en Gert: This show has a lot of characters that are always talked about, but never seen: Bobientje, Marlèneke, Fred Kroket,...
LEGO outright forbid the BIONICLE writers to give away what the Great Beings look like. Since the story was mostly told in written form, this wasn't a problem. In two instances, though, they have gone against the rule — images in the Mata Nui Saga included tiny silhouettes of them, and a pre-existing toy of another character was retroactively revealed to have been a disguised Great Being. The latter is a Double Subversion, since while it's a tangible representation of a GB, it doesn't look like one.
In Peanuts adults were seen exactly once: Some adults were partially shown in an early strip depicting Charlie Brown and Lucy at a golf tournament.
In a few early strips Linus and Lucy's parents (and blanket-hating grandmother) can be "heard" off-panel, and the kids' schoolteachers are heard (albeit as trombone music) in the animated specials.
The Little Red-Haired Girl is also never seen, except once in silhouette. (She is seen in one of the specials, It's Your First Kiss, Charlie Brown, although this was deemed non-canonical by Charles Schulz.)
Also Snoopy's nemesis, The Cat Next Door.
Susie's parents in Calvin and Hobbes. Their voices were heard from off-panel once or twice, but they never appeared.
Susie's mom was seen from the waist down once.
The Phantom is usually wearing a mask or sunglasses; whenever he's not, he's usually shown from behind or has his head out of frame. In-story, anyone who sees his face will die (mainly through Laser-Guided Karma), but his wife and other loved ones are apparently excluded. (In one ancestor story, the current Phantom had died and his son arrived at the Skull Cave to take up the role; he's shown completely, face and all, for several frames. Presumably the curse doesn't take full effect before he dons the suit for the first time, or the comic's readership may have taken a drastic drop in numbers...)
In Cul De Sac, Dil has a horde of unseen older brothers whose hobbies include building trebuchets.
The title characters in George Herriman's early 20th century strip The Family Upstairs. They live on the top floor of an apartment building and cause endless problems for the poor souls situated below them.
This also applied to the title characters in another early strip, Pom Momand's Keeping Up with the Joneses.
In Mutts, Shnelly appears only as ears at the window when Mooch wooes her.
On WWE Smackdown, Tazz frequently referred to a mystery bookmaker named Joey Numbers who gave him insider dirt on the wrestlers currently in the ring.
WWE Raw has an anonymous general manager who only communicates to whomever's in the arena via e-mails sent to Michael Cole.
Recently, it was revealed that the anonymous GM was Hornswoggle.
Vince Russo was this at the beginning of his WCW tenure as "The Powers That Be," and vowed never to show his face. After he was fired and re-hired, he showed his face for the first time in WCW.
On Fraggle Rock, Doc's neighbor Ned Schimelfinny is never shown, and only one side of conversations are ever heard.
Scooter's uncle, the theater owner, was supposed to be this on The Muppet Show. He actually did appear in a couple of season 2 episodes, but the writers decided he was better kept unseen for the force of the gag.
On Sesame Street, Charlie the chef is never show, But it is mentioned by Grover the Waiter.
Bob Newhart was famous for this. Many of his stand-up routines featured a "telephone" bit in which you only heard Bob's side of the conversation and had to imagine the other side. This gag made it into his TV shows a couple of times.
Bob (into phone, while drunk): Derr Bob Hartley. ... Derr. ... Dee, Ar, period.
One who really defies categorization is the character of Vern from any of the late comedian Jim Varney's Ernest routines since the action was always told in first-person fashion from Vern's perspective, allowing the audience to essentially be that character.
Sometimes Vern's hands were seen, such as when Ernest got his hands caught in the window and asks for help, and the audience sees Vern's hands simply closing the blinds.
The late Joyce Grenfell used to do the same thing, with the audience taking the perspective of Shirley, or half the guests at a cocktail party (with Grenfell playing all the other half), or a nursery school.
The unseen Mrs. Grundy, in Thomas Morton's Speed the Plough, in which Dame Ashfield continually worries, "What will Mrs. Grundy say?" of each development. Since then the term "Mrs. Grundy" has passed into everyday speech as the embodiment of prudery and censorship.
Charlie and Myra in Neil Simon's Rumors. Charlie (the host) spends the entire play in his bedroom having shot himself through the earlobe for an unexplained reason, and his wife Myra reveals that she spent the entire evening locked in the basement in the play's final line.
The pooka Harvey in the play and movie of the same name.
Many First-Person Shooters have no cutscenes or anything showing the protagonist, who therefore remains faceless and sometimes also unheard.
The original Doom made this famous - the only reason anybody knows what he looks like is the face in the status bar.
The ultimate example is Gordon Freeman of Half-Life, where we wouldn't even know what he looks like if not for the box. He shows up very briefly in the expansions, but then in Half-Life 2 he doesn't even have a model.
Mental from Serious Sam is this taken the extreme. He's the Big Bad of all the games, and yet all we've seen of him is a bony hand. In a spinoff.
An odd example from Sly 3: Honor Among Thieves: Jing King, the Panda King's daughter, is only ever seen as a silhouette behind a screen. No reason is ever given for this.
Sly's father, in games' flashback scenes, is only seen from the legs down.
Despite being clearly seen in Halo 2, The Gravemind is never seen beyond his tentacles in Halo 3, much to the annoyance of many fans.
It's arguable that we did see him. Some hints suggest that after taking over High Charity, Gravemind grew and merged with it as he infected it; essentially, Gravemind is High Charity at that point.
Mickey Mouse in the first Kingdom Hearts game is absent for most of it, leaving his castle to infiltrate Organization XIII and depending on his trusting aids Goofy and Donald to guide Sora. In the sequel he is vastly more prominent.
Likewise, Sora's family is represented by a disembodied female voice, calling him for dinner.
Gary Smith is absent for the majority of the game Bully. So much so that Jimmy nearly forgets about him and this gives him time to take over the school himself (or at least set it in a frenzy) in all of two hours.
In Backyard Sports, Stephanie Morgan always talks about her best friend Dorothy, who is never seen in the game.
The Lich King is this for the core campaign of Warcraft III, despite being one of the main villains and having his backstory described in detail in the manual (though to be fair, he is a disembodied spirit sealed inside a block of ice, controlling his minions telepathically). He does show up in the expansion.
Until Wrath of the Lich King, this was a staple of most of World of Warcraft. The Big Bad was rarely seen other than his or her particular raid encounter. The Lich King, however, had a very prominent and personal role for the player all throughout Wrath of the Lich King, as does Deathwing in Cataclysm, so this seems to have changed.
In the games Betrayal at Krondor and Return To Krondor, there is a character called the Crawler. The Crawler is never seen or heard, only mentioned through conversations and letters. What is known for certain is that the Crawler is some sort of crime boss, and is assumed to be male. He has an agent named Bear, who is very dangerous on his own. He also has powerful connections (one letter from a powerful man in a land called Kesh warns his niece to "Beware the master of Durbin. The Crawler's plot is a web within a web."). It is too bad a game has not made where you actually get to fight this guy.
The protagonist in the game In the 1st Degree is prosecutor Sterling Granger. The character is male, and you can hear his voice. However, he is never seen in the game.
The Big Bad of The Order of the Stick, Xykon, has a monster that he's keeping in the shadows for the final confrontation with the heroes. When they leave the dungeon, this shadow is provided by a pink Hello Kitty umbrella which, nonetheless, casts a pitch black shadow in which nothing but the monster's eyes can be seen. Even later the monster is placed in a box to preserve the surprise. Rich Burlew is well aware of this trope, and teases the readers by offering small displays of the creature's powers.
Burlew has also confirmed that whatever the creature is, it's not something he thought up.
By the time Gordon Freeman arrives in Concerned, you only get to see his arm (and weapon he's holding.) Sometimes he is in full view, but so far away you can't make out any detail.
In Blip, K's roommate is only shown in silhouette, and never named.
Old Cobbley, Sylvester's (possibly psychotic) homeless friend from ''A Game of Fools is mentioned in passing quite a bit, but yet to make an appearance.
Ssid in Bitmap World. Originally, he was not seen because he was hiding, however, after that storyline, the character is only shown as a pair of eyes under his desk. No explanation is given as to how he manages to get any work done. His first appearance.
In the webcomic Jix, the bounty hunter Maricax is never seen out of his armor, though at the end of his story arc, he is seen wrapped in bandages and some of his burned skin, one of his eyes and the tip of his nose can be seen as well as some green fur poking out of the bandages, but his face itself has never been seen.
Dragon City has a parody of Batman named Batdragon. His face has never been shown in the comic. This is because the comic isn't about him. This allows the reader to see him as the main characters do and none of them know his real identity.
Midnight Screenings has Lettie, Brian Lewis' girlfriend, who "appears" in just about every video he's in, except she's scrunched down in the back seat (or working the camera, depending on how many people are there).
Wade from Kim Possible in person; broken in a late Season 3 episode.
In older MGM cartoons such as Tom and Jerry, humans are only shown from the legs down, with an occasional shaken finger for emphasis.
Cow and Chicken parodied this one — the (human) parents of the title characters didn't exist above their waist. This was only shown twice: in the pilot, and in an episode of I Am Weasel (its spinoff), "Who Rubbed Out Cow and Chicken?" However, their shadows turned up too, and photos of them were of their upside-down legs.
One episode played with this; Chicken, rummaging through the closet, pulls out what appear to be the upper halves of a man and a woman, possibly their parents. After a stunned second, Cow says it's from an old school project.
In the original Peanuts specials, adults were never seen (and only heard as "Mwah-wah" sounds, making them also The Unintelligible), although on at least one occasion the unseen teacher's hand was shown giving a paper to Peppermint Patty.
A few specials and one of the movies showed adults. The Little Red-Haired Girl occasionally put in an appearance, under various designs.
Dr. Claw from Inspector Gadget is never shown except in an over-the-shoulder shot of him sitting in a high-backed chair which obscures everything except his gloved right fist. This was played on in the opening, which had Gadget run in and handcuff him... only to reveal that he's really handcuffed a glove attached to a chair with a bomb in it.
The toy for Dr. Claw does reveal his face... and was a major letdown for many as a result.
When the movie came out, many fans of the show were outraged at their decision to show Dr. Claw's face, as well as his portrayal as a smooth-talking suave businessman who bears no resemblance to the original Evil Overlord. Those who watched the movie first, however, were weirded out by the show's refusal to show his face; that, coupled with his demonic voice, makes him seem kind of silly in comparison to the adapted version.
Things got even weirder with the Direct-to-Video sequel, Inspector Gadget 2. Perhaps as an attempt to appease the fanbase, they switched back to hiding his face, which is downright bizarre for those unfamiliar with the show. Suddenly refusing to show on camera a character who had tons of screentime in the previous movie is, to say the least, confusing when it's unexplained. Since none of the rest of the cast of the first movie return for this one, most viewers assume they're hiding him in a clumsy attempt to cover up the fact that he isn't played by the same person, though they have no problem showing an Inspector Gadget that suddenly doesn't look like Matthew Broderick anymore. Hilariously, the sequel also tries to make Claw's voice more like the original's, and ends up with one that sounds like neither of them. The gap between a smooth, classy and sophisticated sounding Claw and one who sounds like Darth Vader if he needed a lozenge is simply too big to bridge, and they end up with a character that sounds like three different people instead of two.
In the cartoon Hey Arnold!, there is a character that Gerald refers as "Fuzzy Slippers" and who knows a lot about urban legends, the local guys and how to contact a superheroine, but Fuzzy Slippers is never seen.
There is also the boarding house resident Mr. Smith we never see him aside from his silhouette and hand and when they eat together he has Arnold send him his food through a dumbwaiter, one episode has the residents try to break into his apartment so they can see him but Arnold convinces them to respect his privacy after they see he has a photo of the residents whom he considers his family.
Due to bizarre rights issues (as well as a desire to focus on lesser-known heroes), Superman and Wonder Woman were barred from appearing in the first few seasons of Batman: The Brave and the Bold. They were partially seen in one episode, but didn't make proper appearances until the final season. Superman was also usually alluded to via nicknames like "Big Blue" rather than his actual name.
An odd example occurred in the final season of Static Shock. The Teen Titans were mentioned, which was intended as a nod to the team's then-upcoming TV debut. However, Paul Dini was unaware of the fact that Teen Titans was NOT part of the DCAU, and thus the crossover that was alluded to never took place.
One of the stranger examples of this was in the episode "I Oughta Be in Toons" of the Disney series Bonkers, in which Mickey Mouse is the focal point of the plot (he is kidnapped by an impostor who attempts to sign a large contract with a rival studio) but is never actually shown on screen (except for a brief silhouette toward the beginning) or mentioned by name, only heard from inside the cage in which he had been locked.
Disney had a weird, unwritten policy in the '90s that actually putting Mickey Mouse in cartoons would somehow dilute his iconic marketability. The logic was that if anything he was in was ever regarded as bad, it might hurt the whole company, so it was better to never actually do anything with the character at all. A similar policy was instituted for the first Kingdom Hearts game. This weird policy was thankfully dropped in the 2000s, probably because people began to question just what the hell he actually did besides pose for merchandise.
XANA from Code Lyoko. As an artificial intelligence, he solely exists as a program inside supercomputers. Hence he never takes a visible form, but his influence is certainly felt, symbolized by the ever-present "Eye of XANA" logo.
This is arguable as XANA at one point does appear physically, but takes on Jeremy's appearance, and another episode where he appears in Franz Hopper's form. It's debatable that XANA also appears in the form of a black shadow with the logo on it.
The nanny from Muppet Babies is usually only ever seen from the waist down; we never see her face. In a time travel episode we see her whole body but she is bent over while talking to someone in a car so her face is obscured.
In Doug, Principal Buttsavitch was mentioned but never shown. In the "Doug Graduates" (part of the last episode before Disney took over the show, not counting the Christmas special), Doug and Roger actively searched for him so that they could get some words of reassurance, since they were nervous about graduating, but they never actually found him. Also, Skunky Beaumont was a character who was often mentioned but never seen. He appeared in Disney's Doug, and he is portrayed as practically an expy of Jeff Spicoli from Fast Times at Ridgemont High.
Rocko's next-door neighbor/unattainable unrequited crush Melba Toast on Rocko's Modern Life. We only see an arm here or a leg there, and she is never seen at all, only mentioned, after the first season (rather cleverly, the comic book played off of the lack of information about Melba in one issue by having her be a successful model... of products which only require the hand or foot to be displayed).
Most of the characters' parents in Tiny Toon Adventures were like this; if they were shown at all, it was from the waist down. The notable exceptions were Hamton's parents (seen in The Movie), Elmyra's entire family, Calamity Coyote's dad, and Plucky's dad.
In the first two seasons of Rugrats, Chuckie's mom was presumed to be alive, and would be referenced periodically. It wasn't until the Mother's Day episode that she was finally seen, and the revelation came that she was dead.
In earlier episodes Grandpa Lou would often refer to his younger brother "Sparky" whom he had a lot of crazy stories about including one where he supposedly sucked on a bottle for "15 years", we never saw him.
Fire Lord Ozai from Avatar: The Last Airbender isn't shown for the first two seasons. Most of the time he was a shadow behind a curtain or too far to be seen in details, or only his back or legs are shown.
Gazpacho in Chowder often talks about his mom, but she never appears in the series.
Also adults in general they sometimes mention their parents and talk to them but we never see them except in one episode where we see Ed and Eddy's parents' hands drag them away over bad report cards, in the episodes where they're in school the teachers and faculty are also unseen.
Adults do finally appear during a bus scene in the movie finale, but they are only seen from the waists down and none of them actually speak.
Thumper's mother, from the Bambi duology, constantly refers to his father's Green Aesops whenever Thumper is causing her trouble, even though he is never once seen onscreen or even heard in either movie.
Mickey's father the King, from Mickey's The Prince and the Pauper, is never fully shown; even during his death scene only his hands and silhouette are seen.
Beavis And Butthead's moms — they occasionally mention them and sometimes call to them but they are never seen.
The User in ReBoot. The closest we ever get to seeing the human operating Mainframe is a computer screen when he or she restarts the system.
Starswirl the Bearded in My Little Pony: Friendship Is Magic. He's been mentioned numerous times, was responsible for creating spells which fueled the plot of several episodes, Twilight dresses up as him for Nightmare Night, and he's a Distaff Counterpart of an earlier generation character. However, nothing about his appearance or personality is known, which spawned a fan theory that he eventually became Discord.
Completely averted later; he appears (stylized) in a flashback during the season four finale, he's depicted (again stylized) in Three's a Crowd, and he's shown extensively in the comics.
In Futurama Leela would periodically refer to her ex-boyfriend Sean, he finally made an appearance in the episode "Fry and Leela's Big Fling".
King Features' Cool McCool and Terrytoons' James Hound both had the title secret agents with bosses who were only seen sitting in a chair but their faces were never seen.
The Beatles' manager Brian Epstein is shown from the coat collar down at the beginning of the cartoon "Thank You Girl."
An episode of The Simpsons had the main characters temporarily joining a nefarious cult. The actual cult leader, known only as "The Leader", never shows his face, traveling among his disciples in a limousine with tinted windows and making himself visible only by sticking his right arm out the window and waving to his disciples with a white-gloved hand. We don't get to see the inside of the limo until more than halfway through the episode, when the white-gloved hand suddenly grabs Homer and yanks him inside - and the person wearing the gloves turns out to be Marge, who had escaped from the cult's compound earlier in the story. While it's obvious that Marge managed to overthrow The Leader and take his place to help the rest of her family escape, it is never shown exactly how she did this. Yet, it's also subverted because we do see the cult leader's face on big billboards and in the final scene he escapes.
The General on Dastardly and Muttley in Their Flying Machines was never seen but heard over Dastardly's telephone. He pays a visit to the squadron in "Stop Which Pigeon?" but we still only hear him as opposed to see him. Subverted in two comic book stories (Gold Key, Hanna-Barbera Fun-In issues 4 and 7) where he is clearly seen.
In American Dad! we have Francine's older stepsister Gwen whom she would periodically mention and Stan would remark on how attractive she is, she never made an onscreen appearance until the season 11 episode "Now And Gwen".
Muhammad. Muslims have repeatedly threatened violence and murder when Muhammad is pictured in any way.
It's worth noting that this wasn't always the case. This page is a good starting point for understanding this.
It's still a matter of contest among different branches of Muslim faith. Until only a few years ago pictures of Mohammed were perfectly common in many Islamic countries.
Anonymous. Or at least the "true" face of Anonymous (which is everyone and no one, simultaneously). If one must illustrate Anonymous, the two most popular images are: 1) a Guy Fawkes mask and 2) a headless man wearing a suit
Any Strawman Political ideal. An Unnamed Democrat/Republican for example — a very popular choice in the polls for any election, because he/she has no opinions, and therefore has no unpopular opinions.
Karl Albrecht, one of the 10 richest people in the world, is a complete mystery to the public. What is commonly known about him and his brother is that they took over their mothers convencience store and created a business empire with a discount store chain. There is one known photograph of his brother Theo, who also was a billionaire, which was taken in 1971.
This has changed slightly. There are new pictures of both brothers speaking at some event floating around.
John Swartzwelder has written more episodes of the Simpsons than anyone else but lives a notoriously reclusive lifestyle. He is the only main Simpsons writer to not appear on any DVD commentary and is rarely photographed or makes public appearances.
Basically anyone on the internet falls into this trope. By using a variety of names, avatars, and other information, no one is truly who they appear to be and their true selves are never revealed unless that person chooses to do so.
Until the end of World War II, the Japanese Emperor was this to a certain extent. For most common Japanese citizens, they had to avert their eyes when the Emperor passed by, and even the High Command gave their reports to the Emperor with him screened from view. This did not preclude his being filmed and photographed, however.
J. D. Salinger lived outside the public view from 1965 until his death in 2010.
Greta Garbo also refused all interviews and media attention after she left the movie industry in the 1940s until her death in 1990.