He first appeared on DuckTales (to be more exact, the TV movie Super DuckTales, which was later split up into a five-part episode) as a bean counter (literally), but he quit that job to work for Scrooge McDuck as his accountant. Although he is not stupid, Fenton's ambitious ideas and hasty decision-making create serious problems for his boss (from the DuckTales: The Quest for Gold manual, "Fenton can count faster than the speed of sound, but his judgement is not always very sound"). He is doggedly determined to make amends each time by any means necessary, which can create even more complications until he ultimately succeeds, although this requires a large amount of patience with him.
Presumably lacking an education in business and accounting, he tends to take things literally. For example, when Scrooge referred to his large cache of funds as "liquid assets", Fenton has the entire money bin emptied into a nearby lake. Fortunately, he then suggested that his employer "freeze his assets" which inspires McDuck to have the lake frozen solid, then transport the ice to a secure area and recover the money.
Later on, Fenton finds a robot suit called the Gizmosuit, made by Gyro Gearloose. The secret code to activate it is "Blatherskite", a word which Gyro assumes no normal person would use. (Unbeknownst to him, one of Fenton's most frequently used sayings was "blatherin' blatherskite".) As fate would have it, Fenton wanders too close to the suit and utters his favorite expression, thus gaining a secret identity as a superhero. From then on, Fenton always uses "blatherin' blatherskite" to become Gizmoduck, despite only needing the second word — it's possible that he doesn't realize he doesn't have to say the entire phrase.
The only characters privileged to know that Fenton is GizmoDuck are Scrooge McDuck and Fenton's mother, whom he refers to as "M'ma". His mother usually wears a bathrobe and watches television in a rundown trailer-home (and sometimes serves as GizmoDuck as she has the same favorite exclamation as her son). Fenton is in love with Gandra Dee, Scrooge's bean factory receptionist.
As Fenton, he is Scrooge's accountant, but as GizmoDuck, he is Scrooge's personal bodyguard and chief of security for McDuck Enterprises. This gives GizmoDuck an almost Iron Man-like quality, as he plays bodyguard to a rich man. The only difference between the two is that Fenton/GizmoDuck is a wholly different entity from his employer, whereas Iron Man was Tony Stark, who protected his identity by publicly stating that Iron Man was another man who served as Tony Stark's bodyguard.
Fenton also has the surprisingly useful, savant-like ability to count large numbers of objects instantaneously. For instance, that ability allowed him to instantly count the number of shotgun pellets McDuck fired to scare him away in Super DuckTales (because of the gunfire, this scene was cut from syndicated broadcasts). The billionaire, amazed at this improbable skill, then challenged him to determine the value of a collection of coins he flipped in the air and Fenton was accurate to the penny. At the climax of the film, it was Fenton's counting abilities and not Gizmoduck's armor that saved the day; he challenged a massively powerful alien supercomputer, the Master Electronic Leader ("You may call me M-MEL"; voiced by Frank Welker) to a counting contest. Fenton quickly began to pull ahead, causing MEL's circuits to start to overheat in exasperation. Fenton then challenged him to count the number of bolts in a jar. After MEL gave its answer, Fenton responded with a grin, "Sorry MEL, these are nuts, not bolts. Trick question! You lose!" The sheer shock of this defeat drove MEL insane, caused it to explode and began a chain reaction which eventually led to the destruction of the entire robotic planet.
This ability has occasionally led to other swift-processing of information (such as when he determined how to make the Gizmosuit fly while falling from a building), however the only other real talent that Fenton has is that of mimicry; when assuming the role of Gizmoduck, he adopts a deep voice much different from the one he uses in normal speech, which is clearly not the doing of the suit, as he has been heard using his Gizmoduck voice when in his normal identity and vice-versa. He is also understandably bolder, such as coming to the rescue of Scrooge and Mrs. Beakley, both of whom had been unjustly imprisoned, by breaking them both out of jail and challenging the prison guards to open fire on him.
Fenton appeared as a guest star on Darkwing Duck on occasion as Launchpad McQuack's old friend from Duckburg as both identities. However, his suit seemed to be smaller than the one from DuckTales and lacks the black coloring in the upper arms (this may be due to the fact that in the DuckTales episode "Attack of the Metal Mites", his suit was eaten by Flintheart Glomgold's metal-mites, so in Darkwing Duck, he probably uses a different one).
First appearing in "Tiff of the Titans", Fenton travelled to St. Canard to prevent the theft of the prototype E.G.R.E.T. assault vehicle by Steelbeak and F.O.W.L. under the guise of visiting Launchpad. In both his Fenton and Gizmoduck personas, Fenton proved to be an annoyance to both of Darkwing's personas by being an annoying house guest to Drake Mallard and mistaking Darkwing for a criminal. After nearly being drowned by a Yolk bomb dropped by Steelbeak, the two joined forces and succeeded in stopping Stealbeak from escaping with the E.G.R.E.T.
In his subsequent appearances, Gizmoduck developed a complex friendship/rivalry with Darkwing and even joined Darkwing's Justice Ducks team where he deferred leadership to Darkwing.
Originally, GizmoDuck was intended to be an artificially intelligent robot (and, indeed in the series, Scrooge originally intended to use a security robot designed by Gyro with the designation of G.I.C.U.2 until he realized it "didn't have a brain", as it was so mechanistic that it wouldn't even let Scrooge himself into his money bin), but the idea was later changed to the current one, akin to Iron Man or RoboCop.
GizmoDuck was originally called RoboDuck, which, coincidentally, is his name in Japan and also explains the "R" shaped insignia on his chest.
Appearances in other media
Strangely, Fenton never appeared in either of the original two DuckTales comic book series, despite continuous letters asking that he appear. He did, however, appear in the comic stories printed in Disney Adventures. One story, "A Switch in Time", had him and Launchpad switching jobs for a day. Another story had Huey, Dewey, Louie making their own Gizmosuit and bickering amongst themselves over who should get to wear it, as Fenton is looking to retire from his career as Gizmoduck as assessing a successor. The three nephews get their comeuppance when Fenton says he has decided on the most mature of the bunch: Webby, causing the boys to express shock at imagining a "girl Gizmoduck".
When DuckTales and Darkwing Duck were revived for the Disney comics line by Boom! Studios, the fan requests were finally heard and Fenton returned to comic books in the "Dangerous Currency" story arc.
Gizmoduck had a prominent cameo appearance in the first DuckTales video game on the NES. In the Moon level, in order to break down a barrier blocking the way to the boss of the level, Scrooge has to find a remote control on the UFO, and use it to signal Gizmoduck so he can blast the barrier down. The HD remake expands on Fenton's role in the level, in that he actually gets kidnapped by the UFO and Scrooge has to venture into the UFO to save Fenton and also find all the parts of the Gizmosuit.
Media: DuckTales (Episode list) | Videography | Gladstone comic | Magazine | Video game (HD remake | Soundtrack) | Disney Comics | DuckTales 2 | The Quest for Gold | DuckTales the Movie: Treasure of the Lost Lamp | Raw Toonage | Boom! Studios comic | Scrooge's Loot
Heroes and Allies: Scrooge McDuck | Huey, Dewey and Louie | Launchpad McQuack | Webby Vanderquack | Betina Beakley | Duckworth | Gyro Gearloose | Doofus Drake | Fenton Crackshell | Donald Duck | Bubba the Cave Duck | Tootsie the Triceratops | Gene the Genie | Glittering Goldie | Admiral Grimmitz | Duchess of Swansylvania
Villains: Beagle Boys (Big Time, Burger, Bouncer, Baggy, Bankjob, Bugle, and Babyface) | Ma Beagle | Flintheart Glomgold | Magica De Spell | Poe De Spell | El Capitan | Merlock the Magician | Dijon | Cinnamon Teal | Dracula Duck | Armstrong
Organizations: Junior Woodchucks
Heroes and Allies: Darkwing Duck | Launchpad McQuack | Gosalyn Mallard | Honker Muddlefoot | Tank Muddlefoot | Herb and Binkie Muddlefoot | Gizmoduck | Morgana McCawber | Neptunia | Stegmutt | J. Gander Hooter | Derek Blunt | Vladimir Goudenov Grizzlikof | Comet Guy
Villains: Negaduck/Negatron | Megavolt | Liquidator | Bushroot | Quackerjack | Steelbeak | Taurus Bulba | Hammerhead Hannigan | Hoof and Mouth | Tantalus | Clovis | Tuskernini | Dr. Fossil | Phineas Sharp | Professor Moliarty | Ammonia Pine | Paddywhack | Lilliput Gooney | Jambalaya Jake | Gumbo | Bug Master | High Command | Major Synapse | Hotshot & Flygirl | Splatter Phoenix | Camille Chameleon | Anna Matronic | Isis Vanderchill | Johnny T. Rex | Fluffy | Dark Warrior Duck