I got a bunch of the comics, including the first few volumes from the 1960s in hardcover while in Rome about 15 (?) years ago. Like proof said, he's basically lifted from Fantomas (those are fun reads if you get the chance) and created by two sisters, which is even more unusual because of how irredeemable the character was. I mean, a thief, yeah, but also a murderer and rapist. And on that note, another thing that's interesting is how closely early Lupin III mirrors Diabolik. Some of the early Lupin stories are very close to the first volumes of Diabolik. I just don't know if Monkey Punch would have had any exposure to Italian comics in the 1960s. The characters also had very similar trajectories in the public consciousness, both starting very hard-edged, with evil protagonists who were diluted over time to eventually become child-friendly.