Backstory
Where Lupin is a personification of Europe's wit and sophistication and Goemon signifies the discipline and quiet dignity of Japan, Jigen seems to be rough and tumble all-American. Which country he hails from, and for that matter his ethnic origin, have never been made clear. He has dropped hints from his love of US food, his gun preference, and remarks made about "buying US Saving Bonds" (The Mystery of Mamo) that he is a expatriated American residing in Japan. In the 2002 OVA Episode 0: First Contact, he is described as a New York based trigger-man employed by a mafia bigwig. Assigned to eliminate Lupin, he instead finds the Frenchman more entertaining and partners up with his former target without hesitation.
In the manga, Jigen first appears rather abruptly in the seventh chapter as a major character in one plot arc. He soon begins to appear more extensively, teaming up with Lupin opportunistically in several capers, opposing Lupin as often as not. By the end of the first series, however, he has firmly settled into place as Lupin's most trusted ally. As Monkey Punch never really gave any of the characters a backstory, different film makers have their own take on it. In the 1978 film The Mystery of Mamo (originally titled simply Lupin III), he says both he and Goemon follow Lupin around because they've both vowed to kill him.
Read more about this topic: Daisuke Jigen