Sequential Artist

Most comic book artists, draw comic books. As a result, it's a common occupation or hobby for characters in media. These characters are typically very Genre Savvy or Wrong Genre Savvy and possible Fourth Wall breaker. Most often they are aspiring Comic Artists / Mangaka who work on Comics / Doujin / Manga to sell at Fan Conventions. Sometimes these can take the form of an Author Avatar relating problems they had or have making their work. They tend to have a penchant for Cosplay generally as the characters in their own works, or works they're fans of.

In Japan they are more often Female and are often Yaoi Fan Girls. Males are far more common in the Western world thanks to the western idea that Comic Books are for boys.

A common Super Power for these characters is Art Initiates Life.

Examples of Sequential Artist include:

Anime and Manga

Mahou Sensei Negima, Haruna is an aspiring artist who draws Doujins of the popular Magical Girl anime Mahou Shojo Biblion. And the thing she hates the most? Deadlines.
Hayate the Combat Butler, Nagi is an aspiring artist and keeps trying to win the Rookie award for a magazine. However, her manga are completely nonsensical.
Sayonara, Zetsubou-sensei, Harumi Fujiyoshi specifically is a Yaoi Doujin artist with an obsession with pairings. First seen drawing by hand but by season 2 she upgrades to digital.
To LOVE-Ru, Yuuki Rito's father Saibai is a professional manga artist who treats every page like a battle.
- Zastin, bodyguard of the alien princess Lala, becomes Saibai's assistant and decides he wants to become a professional mangaka himself. Lala's father, the emperor of the universe, doesn't appreciate this.
There are a few mangaka characters in 20th Century Boys, whose philosophizing about manga is a reflection of the author's own opinions.
Bakuman｡'s entire premise is this.
Hanaukyo Maid Tai La Verite episode 5. Ikuyo Suzuki reveals that she's a mangaka and takes the main characters to Comiket to sell her Doujinshi, which features a (fictional) relationship between the maids Yashima Sanae and Konoe Tsurugi.
Sora Wakanae of Family Compo is a professional manga author. At least three story arcs revolve around his profession.
Hiyori Tamura from Lucky Star, who draws Doujinshi.
Aki Hinata in Sgt Frog is a manga editor, and thus will often make reference to manga tropes, and uses the Keroro Platoon as inspiration for new manga.
Doujin Work is about some girls trying (and failing) beeing succesfull as doujinshi artists.
Genshiken has 2 specific arcs about this as they attempt to complete Doujin for Comiket. Ougi is specifically a Yaoi artist who makes her boyfriend an aspiring Editor edit it (Most definitively not a yaoi fan...)
The protagonist of I Am a Hero works as an assistant manga artist in the beginning of the series, though he strives to one day become a professional mangaka in his own right. These plans are interrupted by an ill-timed Zombie Apocalypse, however.
Hanasaki Sono in Sensitive Pornograph is an uke older than his partner. Oh, and the creator of delicate, lightly fanservicey girl manga.
Even a Monkey Can Draw Manga tells the tale of a loser aspiring to draw a successful manga. It's a parody of "How to draw manga" books, the Japanese comic industry, the conventions and cliches of manga, and generally very apt.
The main character of Free Soul, Keito, is an aspiring mangaka.

Comic Books

Green Lantern Kyle Rayner is a professional comic book artist and writer. If he's fighting alongside other Lanterns, this tends to be shown as influencing his style of combat.
Steve Rogers (aka Captain America (comics)) is a professional illustrator and cartoonist, and at one time was the writer/artist for the in-universe Captain America comic book.
Chuck Clayton, a friend of Archie Andrews of Archie Comics fame, is a cartoonist. His greatest ambition is to become a comic book artist after he graduates from Riverdale High School.
Scribbly, the main character in what would later become the Golden Age Red Tornado strips, was a young boy who wandered around his neighbourhood drawing cartoons of what was happening. In the later DC mythos, Scribbly went on to work for a newspaper as a cartoonist. Granted, the strip was meant to be semi-autobiographical...
After Alan Moore retooled Supreme as a homage to Superman, the Clark-Kent-equivalent character was a comic book artist, and the Lois Lane and Jimmy Olsen equivalents were comic book writers.

Film

In Secondhand Lions Walter grows up to be a comic strip artist, with a strip called "Walter and Jasmine", about a boy and his pet lion, which he based on his own experiences as a child.
In Chasing Amy (and Jay and Silent Bob Strike Back), Holden McNeil and Banky Edwards are writers/artists for Bluntman and Chronic comic, Alyssa Jones is writer/artist for Idiosyncratic Routine, and Hooper X produces the comic White Hatin' Coon.
In Freddy Got Fingered, Tom Green plays a cartoonist named Gordon Brody.
Michael Kellum of Three Men and a Baby is a professional cartoonist. It is his artwork that decorates the hallway outside the apartment the three men share.

Newspaper Comics

Rerun van Pelt from Peanuts, younger brother to Lucy and Linus, is an "underground cartoonist" who is apparently quite the subversive even though he's only in kindergarten.
- Also from Peanuts, Charlie Brown has tried his hand at producing a strip. Nobody he shows it to ever likes it.
Jon Arbuckle from Garfield is officially a cartoonist, but he's almost never shown doing his job.
The father in the somewhat-autobiographical family in The Family Circus is a cartoonist.

Live Action TV

The Criminal Minds episode "True Night" features a serial killer whose crimes seem to match up with the work of a comicbook artist.
The Bones episode "The Superhero in the Alley" features a teenager who draws his own comic books.
Isaac Mendez of Heroes used his precognitive Psychic Powers to write and draw his comic book, 9th Wonders.
Tim Bisley of Spaced is writing and drawing a graphic novel... but never seems to get anywhere with it.
Caroline in The City focused around the life of cartoonist Caroline Duffy.

Web Comics

Ménage à 3 centres around Gary, a comic and manga fan who is attempting to make a doujinshi with the help of Yuki, the daughter of a famous hentai artist.
Arthur, Merlin and Nimue are all webcartoonists in the modern arc of Arthur, King of Time and Space.
Sarah from El Goonish Shive has at least two comic strips she's working on. One is entitled "The Wizard Sarah", and the other features a squirrel-boy named Dan.
Sequential Art focuses on Art a Sequential Artist.