Mister Descriptor

Characters with only a surname, which describes them.

Damien: What is your name?
Elliot: Um, well, what with me being a cat and all... Cat?
Damien: All of you animal people have horrible names.

Elliot: Well, uh, what can you expect? We were named by scientists, not writers or cartoonists.

—El Goonish Shive

Sister Trope of Something Person.

If the word for their surname doesn't have any obvious relation to the character in question, it's Mister Strangenoun. If it's not a character but an offhand description of a prop, look toward I Call It Vera (or the demonstrative I Call Him "Mister Happy"). If their name reflects their color, it's also Colorful Theme Naming.

Examples of Mister Descriptor include:

Comic Books

Mr. Men
The Riddler of Batman, born Edward Nashton, legally changed his surname - making him E. Nigma.
Mr. Sinister of X Men fame, who took that name because it was the last word his wife said to him as she died, when she realized what a monster he was.

Film

Ditto Reservoir Dogs.
Mr. Furious in Mystery Men.
Mr. Glass from Unbreakable.

Live Action Televsion

Mr. Big in the first episode of Get Smart.

Literature

Mr. Pump in the Discworld novel Going Postal. Of course, he's a Golem who was employed pumping water at the bottom of a well for a few hundred years, so it's an apt description (his 'name' was previously Pump 19)

Radio

Mr. Show with Bob and David

Tabletop Games

The characters in Clue are named after the color of their pieces: Mr. Green, Mrs. White, Ms. Scarlet, Mrs. Peacock, Colonel Mustard, and Professor Plum.

Theatre

The song "Mister Cellophane" from Chicago.

Web Comic

El Goonish Shive chimerae are usually called by some reference to their "beast" part (like Nioi/Kaoli with a Bilingual Bonus). So despite Elliot being horrible at lying his captor mistaking him for one of these didn't even doubt:

Damien: All of you animal people have horrible names.
Elliot: Well, uh, what can you expect? We were named by scientists, not writers or cartoonists.

Web Original

Invoked with the Mister Collector series from the SCP Foundation. They were built to be that way.

Western Animation

From South Park, Mr. Garrison's puppet Mr. Hat, followed by his replacements, Mr. Stick and Mr. Slave.