A character may strictly be He Who Must Not Be Seen, but they need to transmit orders to their subordinates and intimidate their enemies.

The solution? Hire this guy. His job (although sometimes not his only job) is to talk for (and sometimes even impersonate) the real He Who Must Not Be Seen, who often is standing somewhere in the background.

The Mouth Of Sauron often serves as The Dragon, and can sometimes be a villainous counterpart to The Aragorn: when the real villain is in the background pulling strings, he needs someone to go out and lead his Evil Army against the forces of good.

Sometimes, the Mouth Of Sauron is set up to be the Big Bad and is even believed to be so by everyone except the real Big Bad's most trusted advisers. In this case, it overlaps with The Man Behind The Man.

While many examples and the Trope Namer are villainous, this is not necessarily an "evil-only" trope: any heroic Mysterious Employer is likely to have one.

Rolling Updates, Needs A Better Description

Examples:

Anime And Manga

• In Death Note, the unseen "Kira" at first only attracts a widely scattered cult following, but as his influence grows and he becomes a Villain With Good Publicity, it becomes necessary for Kira to select an official representative. The position is most successfully filled by Kiyomi Takada, who also passes secret messages between Light and Mikami and is actually entrusted with the killings for a while.

Comic Books

• Lewis Prothero in V For Vendetta is the Voice of Fate, the person who speaks on behalf of the government.

Film

• Metatron in Dogma is the Voice of God.
• Darth Vader in Star Wars, for Big Bad Palpatine.
• Mr. Kobeyashi from The Usual Suspects.

Literature

• The Mouth Of Sauron from The Lord Of The Rings, who serves as Evil Overlord Sauron's herald and lieutenant.
• Walker, from Simon R. Green's Nightside series is the Voice of the Authorities, the hidden rulers of the Nightside.
• Metatron and Beelzebub for Good Omens.
• In the Wheel Of Time series, Shaidar Haran (nicknamed "Superfade" by fans) acts as the mouth of the Dark One, never getting personally involved but giving direct orders (and threats) to the Forsaken.

Live Action TV

• Bosley from ~Charlie's Angels~, for Charlie... though the phone is a more literal example.
• This is effectively Richard Alpert's job on Lost: to act as a representative for Jacob so Jacob doesn't have to interact with anyone directly.
• Eve and Marcus in Angel Season Five.
• Mr. Morden in Babylon Five.
• Felix Dreifus acts as this for Management in Carnivale.

Tabletop RPG

• In the Back Story of In Nomine, the angel Metatron acts as the voice of God. Lucifer kills him, starting off the events of the Fall.

Video Games

• The Collector General in Mass Effect 2 serves this role for Harbinger.