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 GizmoDuck made by Gyro Gearloose. The code word 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 "blathering 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 "blathering 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 security guard and personal 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 ability, 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" 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. 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.
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.
Fenton has 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 - the possible explanation is that in DuckTales finale his suit was eaten by Flintheart Glomgold's metalmites, so in Darkwing Duck he probably uses a different one. He appears more as GizmoDuck than himself, and is a superhero rival of Darkwing's.
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 until he realized "it didn't have a brain" — it was so mechanistic 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.
Strangely, Fenton never appeared in the DuckTales comic book series, despite continuous letters asking that he appear. He did, however, appear in the comic stories printed in Disney Adventures, mostly showing up in Darkwing Duck stories, as well as the DuckTales video game for the Nintendo Entertainment System.
GizmoDuck (along with Bubba and an unused character named Alien Duck) were created by Tad Stones around the same time Chip 'N' Dale were added to Rescue Rangers. Gizmo Duck was originally called Robo Duck, which coincidentally is his name in Japan and also explains the "R" shaped insignia on his chest.
The suit contains a built-in video screen, which Fenton uses for communication, although his television-addicted mother finds another use for it after the two discover that the suit can be deactivated with a television remote control.
As Fenton doesn't quite know how the suit works (especially in the earlier episodes), one last-ditch battle tactic that he relies on is to push all of his buttons, which has very unpredictable (but humorous) results, usually ending in victory (although leaving Fenton battered and bruised). Hence this technique can be considered double-edged.