Phantom 2040 was designated as the 24th Phantom in the line, so that would make him the great-grandson of the current 21st Phantom. But since 2040 is a mere 15 years away, that isn’t accurate anymore. I think it was a missed opportunity not to age The Phantom in real time. Since he’s been established early on as a legacy character, it would have worked.
But, hey, a lot of Phans are happy that the current Phantom is still the same one from the 1930s, so what do I know.
