Deadpool (aka Wade Winston Wilson) made his debut appearance in The New Mutants #98 (1991).
Initially Deadpool was depicted as a supervillain when he made his first appearance in The New Mutants and later in issues of X-Force, but later evolved into his more recognizable antiheroic persona. Deadpool is a disfigured and mentally unstable mercenary with the superhuman ability of an accelerated healing factor and physical prowess. He is known as the "Merc with a Mouth" because of his talkative nature and tendency to break the fourth wall, which is used by writers for humorous effect and running gags.
Deadpool is the brainchild of the artist Rob Liefeld. Being a fan of the Teen Titans, Liefeld conceptualised the character with reference from Deathstroke, and Spiderman from his love to the superhero. Storywriter Fabian Nicieza gave the character his loudmouth personality, and named him Wade Wilson, after Deathstroke's name of Slade Wilson as a joke between him and Liefeld.
At the preliminary stage, Liefeld had wanted to model Deadpool after Wolverine, another of his favourite character. With which, he initially appeared as an aggressive assassin hired to take down Cable and team.
After some cameos in several titles, the mercenary received his own comic book as a mini-series in 1993 and was an immediate success. After another subsequent test-water series, the character finally received his very own ongoing title in 1997 and anchored his personality as a comedic antihero in an action-filled story plot.
The character's non stop witty sarcasm and loud humour amidst the battles gained so much popularity, (like many evil businesses) more likewise characters were being created to milk the moolah off the readers.
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Backtracking the character's intro, Deadpool was professed to be Canadian, growing up in Clair, Saskatchewan, and went to school in Wadena, Saskatchewan.
The original story had him joining the Weapon X program after being kicked out of the United States Army Special Forces and given an artificial healing factor based on Wolverine's thanks to Dr. Emrys Killebrew, one of the head scientists. It was also through this experiment that Wilson got his whole body scarred and facial disfigured.
Deadpool is depicted as having an accelerated healing factor, which not only prevents him from being permanently injured through enhanced cell regeneration throughout his body, but also causes psychosis and mental instability, as his neurons are also affected by the accelerated regeneration. It is thought that while his psychoses are a handicap, they are also one of his assets as they make him an extremely unpredictable opponent.
In all, the character is so popular, at one time, almost every other title had a Deapool variant tied to them. How's that for an immortal?