The series will end with Duke Togo killed in a duel with a rival assassin
Given that Golgo has met quite a few enemies that were able to give close calls to him, it's inevitable that he might eventually find one who can actually best him. Before dying though, Togo will still manage to take him out in a mutual shot with him smoking a cigarrete, confirming his accomplishment on his end of the contract and passing on. After all, he always finishes his contracts. Golgo has high-functioning autism.
He does not display emotions in a normal manner, he is extremely talented at one thing and everything pertaining to it, he can hyper-focus on an objective, and he gets annoyed if he isn't allowed to do things his own way. Hence, he doesn't retire despite his enormous successes and him not truly enjoying his job because he wouldn't know what to do if he ever stopped. There is no one "Golgo 13".
It's a Dread Pirate Roberts-type situation. The previous Golgo grooms a successor to take over once he dies, gives him a bit of Magic Plastic Surgery, and brainwashes said successor into having the memories of his predecessors. Said memories not only include Golgo's shooting expertise, but also the secret of his origins. This also explains why Golgo's so twitchy on his backstory: EVERY supposed Golgo origin story is in fact correct and applies to their corresponding Golgo.- Or if each new Golgo isn't brainwashed, they kill to protect the identities of the other Golgos.
