Golgo 13 is a genetically-enhanced clone, the culmination of decades of secret research into gene augmentation. His creation was funded by a cabal of criminal masterminds who donated their own DNA to the project.
In the 1940s, five men of various nationalities served in the same unit of the French Foreign Legion. After their stint was finished, four of those men returned to their respective countries and founded their own criminal empires. The remaining one, Dr. Ort-Meyer, ran a mental institution, which he used as a cover for genetic experiments. In exchange for research funding, Ort-Meyer provided his former comrades with donor organs harvested from clone bodies, which significantly extended their longevity. (By the events of Golgo 13: The Professional, each of Ort-Meyer's accomplices were well into their 60-70s, but appeared 10-20 years younger.)
The "Five Fathers", as they are referred to in the series, are the the gentlemen whose DNA went into Golgo 13's creation. The first is Otto Wolfgang Ort-Meyer, a discredited German scientist living in Romania. His radical theories were deemed insane by his peers. Ort-Meyer believes that genetic recombination and human cloning could be used to produce a perfected version of the human species, superior in strength and mind and unburdened by conscience. His ultimate goal (unbeknownst to his associates) is to create an army of flawless and unquestioningly obedient supermen. When Ort-Meyer's associates become suspicious of his motives, he employed Golgo 13 to systematically eliminate each of them. When Golgo 13 discovered Ort-Meyer's plans, he managed to eradicate his latest line of clones and break his neck.