As a relative newcomer to Lupin I've been curious about a lot of things. To me answering some of my curiosities helps me to enjoy Lupin even more.

This question might be simple, or not. Anyhow, here's the current question in my wee-little brain.
Main Characters:
Daisuke Jigen
Goemon Ishikawa
Fujiko Mine
Kōichi Zenigata
Arsene Lupin III
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Is there any rhyme or reason why some folks are referred to by their last name, but others by their first name? Is there a Japanese cultural rule that explains it? Something else? Or is it just random?

It could be argued that Lupin's name has become mononymous over time, even to those who aren't avid fans. And Zenigata is pretty easy to figure out why his last name is used, but as for the rest of the gang, I wonder why some go by their last name and others go by their first name.