Wilson Fisk claims he is the greatest example of a self-made man in the world. This is, to put it kindly, hogwash. Wilson Fisk is a beneficiary of numerous lucky breaks and what he couldn't gain through sympathy, he stole through force. Growing up a poor fat Caucasian boy in Hell's Kitchen, Wilson Fisk was bullied repeatedly until he became inspired by a brief glimpse of a Sumo match on a store television. Doing research on this art, Wilson Fisk changed his deit and transformed himself into man whose body more closely resembled a football player than a tub of lard. Swiftly learning the joys of bullying himself, he dropped out of High School and became a full time drug dealer.
Wilson Fisk might have lived and died in obscurity, were it not for the misguided sympathy of Don Rigoletto. One of New York's 'Old-Time' crime lords, Don Rigoletto unofficially adopted Wilson and forced him back into school. Wilson Fisk became his bodyguard and swiftly became acquainted with levels of power he'd never dreamed of. Had Wilson Fisk an ounce of loyalty in his body, he would have stood by Don Rigoletto and recognized the man's attempts to get him off the streets. Unfortunately, Wilson Fisk had no code of honor and snapped the neck of the Don in order to become leader of the man's gang. Wilson Fisk promised the Don's underlings a vast collection of drug profits and delivered.
Wilson Fisk managed to survive being a crime lord in New York City despite his lack of a pure-blooded Italian pedigree by making deals that no "respectable" boss would. Asians, Blacks, Russians, and other ethnic gangs were brought into the Kingpin's organization. He deliberately targeted families and brought down the law on his competitors. Whatever weaknesses the Kingpin saw in his enemies, such as mercy or compassion, he ruthlessly exploited. Ironically, Wilson Fisk would have never become ruler of the New York City underworld if not for the fact that Spiderman defeated several of his early competitors. Individuals like the Big Man and the Owl were beaten before they could muster an early resistance to the Kingpin's growing empire.
For much of the 90s, the Kingpin had a wave of good fortune. The collapse of the Falcone family in Gotham City and Intergang's troubles with Superman gave the Kingpin an unprecedented chance to expand on a national level. The Kingpin also learned to make friends with important national figures. Lex Luthor happily built up his company by investing the Kingpin's billions and relied on the man's network to dispose of unwanted troublemakers. The Green Goblin also willingly passed along industrial secrets in exchange for the occasional "favor" like killing nosy reporters. For a short period, much of the United States of America was secretly under Wilson Fisk's corrosive influence.
Then things got personal. Spiderman proved surprisingly incorruptible and was impossible to kill. The Punisher couldn't wipe out crime in New York City but he killed possibly a third of the Kingpin's most experienced personnel during his initial career. But if you honestly want to know who destroyed the Kingpin's criminal empire then you only need to know two names. Richard Fisk and Matt Murdock. The Kingpin's own son and an individual that he initially dismissed as a second-rate Spiderman.
Richard Fisk was a man of questionable morality and severe emotional problems but he was even smarter than his father. Motivated by a misguided sense of justice, the Rose shattered any illusions of invincibility that the Kingpin had. Wilson Fisk also realized that he had real feelings for his son and couldn't kill him. His previous policy of being utterly without weakness crumbled before the discovery that he loved his family. As for Matt Murdock, the Kingpin has since realized that Spiderman was fundamentally a moral man above all things. Daredevil, on the other hand, is capable of things that Spiderman would never stoop to.
Eventually, the Kingpin left the United States of America with his wife and what liquid wealth he'd put away for such an occasion. His criminal empire was totally destroyed by Matt Murdock's revenge for the death of Elektra and humiliation of Karen Page. Ironically, he arrived in Hong Kong just when the insidious Doctor Fu Manchu's criminal empire was without a leader. With a good deal of lying, the Kingpin managed to acquire nearly all that man's former assets. The Si-Fan would never follow a white man like they did their master but were entirely happy to have someone coordinate their efforts until their master's expected ressurection. Ra's Al Ghul, the secret master of Fu Manchu, was having his own problems and didn't consider the Kingpin honestly worth his attention.
Since that time, Wilson Fisk has slowly become solvent again. His attempts to retake the New York City Underworld, however, have been miserable failures. He's even served prison time, something that he never honestly expected could happen. Worse, his wife Vanessa killed their son Richard after their last attempt to reconcile failed miserably. While Wilson Fisk would never admit it, he no longer cares for his wife the way he once did. She's become tainted with his business in a way that he never wanted. Vanessa, for her part, is confused by her husband's sudden lack of affection. After all, she killed Richard for him.
The Kingpin still wields exceptional authority across the world and is slowly building bridges with HYDRA, the Maggia, Intergang, and even H.I.V.E. He's renewed his old associations with Lex Luthor and the Green Goblin, despite their own up and down successes. He privately knows he'll never attain the former fear and grandeur that he had as the most powerful crime boss in America. However, the Kingpin is still standing. As long as that is true, he's still dangerous.
The Kingpin was previously considered a New York City criminal and mostly left alone by the major players in the superhero world. Only Batman genuinely suspected that he was as dangerous as he really was. His willingness to dabble in the creation of supervillains has brought him more attention than is perhaps prudent. Unfortunately, despite his checkered history with superhumans, it's also more profitable than costly. Bruce Wayne would dealy love to take down Wilson Fisk permanently but hasn't yet figured out a way to extricate himself from Gotham City's problems long enough to put the man behind bars. The heroic Shang-Chi, master of Kung-Fu, is also interested in having words with the new master of the Si-Fan.
And by X-Men, I don't mean all the new ones introduced over the years. The original 5, plus the likes of Shadowcat, Rogue, Wolverine and such. Look at the characters on X-Men: Evolution for the X-Men I would love to see detailed in this universe. 
