Characters known only by their title

[Hot Pepsi]

The Doctor
The Dude
The Mandolorian and The Child (until later on in the story)
Ed Norton is just "the narrator" in Fight Club, I think.
The Smoking Man
The Skipper and The Professor
Fleabag

[3 Colours Red]

Nibbler?

"In the time it would take to pronounce one letter of my true name, a trillion cosmoses would flair into existence and sink into eternal night."

[Lobachevsky]

Dr No

[Hot Pepsi]

Nibbler?

"In the time it would take to pronounce one letter of my true name, a trillion cosmoses would flair into existence and sink into eternal night."

Lord Nibbler, right?

[caja-dglh]

The Emperor

[Amor de Cosmos]

'The Monster' (Frankenstein in various editions)

'The Creature' (of The Black Lagoon)

'The Gimp' (Pulp Fiction)

Do we ever know the name of 'The Joker' in any of the Batman iterations?

[Hot Pepsi]

In some iterations, yes, but it’s not canon.

[Sits]

The Master (Dr. Who)
The Hood (Thunderbirds)

[3 Colours Red]

The Janitor (Scrubs)

[Hot Pepsi]

His name is Glenn Matthews, but on the show, even when that’s revealed, nobody was sure if it was supposed to be true.
https://en.m.wikipedia.org/wiki/Janitor_(Scrubs)

[Patrick Thistle]

Pilot (Farscape)

[Guy Profumo]

Nibbler?

"In the time it would take to pronounce one letter of my true name, a trillion cosmoses would flair into existence and sink into eternal night."

Flare
(Pendant)

[delicatemoth]

The Smoking Man

X Files? Wasn't he specifically 'Marlboro Smoking Man'?

I remember the anti-corruption cop in the 'Red Riding' books as being 'The Hunter', but not sure if this was used in the TV adaptation.

Probably quite a few of these in Brecht plays.

[Guy Profumo]

Brigadier Sir Alistair Gordon Lethbridge-Stewart was always referred to as "The Brigadier" and addressed as "Brigadier" (following his promotion from Colonel) wasn't he?

[Sits]

Yes

[Jah Womble]

Woman Who Urinates Herself

(Admittedly, it wasn’t the most fully-developed of characters...)

[Guy Profumo]

The Janitor (Scrubs)

Just "Janitor", surely?

No definite article

(He's canonically known as Neil Flynn and Glenn Matthews - his other alter egos are fictitious personae he's invented for himself)

[Nurse Duckett]

Friends - The Ugly Naked Guy

[elguapo4]

Mr Big

Trigger

Captain Sweatpants

[Hot Pepsi]

Just "Janitor", surely?

No definite article

(He's canonically known as Neil Flynn and Glenn Matthews - his other alter egos are fictitious personae he's invented for himself)

Also, Dr. Jan Itor.

[Discordant Resonance]

HAL is technically the brand name of the computer, and thus a title, rather than a personal name.

[Baptiste]

Number 2 and Number 6

[Guy Profumo]

Also, Dr. Jan Itor.

No, another sobriquet he appropriated for himself
Along with Tommy and Klaus

[Walter Knight]

Trigger

I've read online that Trigger's actual name was Colin Ball, but am not sure if it was ever 'revealed' as such in OFAH (assuming you mean that one and not the horse)...

[Southport Zeb]

I've read online that Trigger's actual name was Colin Ball, but am not sure if it was ever 'revealed' as such in OFAH (assuming you mean that one and not the horse)...

It's not mentioned in OFAH but is in the prequel, Rock and Chips