Agent X is a former Deadpool enemy and a citizen of the nation of Krakoa.
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History
Nijo was an assassin, mercenary, and agent employed by the telepathic German assassin called the Black Swan. He blamed Deadpool for his brother's death as he was among the targets of Deadpool's apparently successful assassination of the Japanese crime lords named the Four Winds. After Black Swan downloaded a mental virus into Deadpool's brain, Deadpool came for both Swan and Nijo at the Swan's castle in Germany. However, Black Swan revealed he had in fact killed the Four Winds, including Nijo's brother, and stabbed him out of boredom with his ranting and disobedience. His vendetta with Deadpool was that the hits had erroneously been credited to the wrong man. A bomb Deadpool had brought went off, and all three were presumed killed.
All three had partially merged during the explosion when Black Swan used his powers in an act of desperation to save himself. Black Swan psionically acquired Deadpool's healing factor but accidentally made contact with Nijo and their personality traits got mixed around. Swan also gave Nijo's corpse Deadpool's regenerative powers, restoring him to a form of life five minutes after his death. As well as part of his personality, Nijo also gained Swan's firearms talents, ambition, some of his refined tastes, preferences and biases, and also the bodily scarring and outrage he'd received from Deadpool throwing him face first into a white hot fireplace griddle at his home.
Nijo soon reemerged as Alex Hayden in New York with one goal in mind - to be the best mercenary alive. He was found by Sandi Brandenberg, Deadpool's former secretary, who assumed he was Deadpool by his powers and demeanor. He subconsciously took the name as a combination of composers he liked: Alexander Grieg and Josef Haydn, which Sandi worked out through his fondness for classical music.