Well, agreed that a good story usually, has a good hanful of main characters, but that can have its setbacks. As I'm sure if anyone takes a good number of mangas/ animes/ books/ etc that most ost the time, most of the characters come from the same cookie cutter mold, usually evolve the same way, and even if some events transpire differently, they all usually end up the same. Of course its good to do what's tried and true. But I think what makes Gantz unique is two things, one that it keeps the reader guessing because you can't rest easy with the characters even if they have a great amount of resolve( even Kurono himself almost died in the Buddha massacre even though it could be argued that as the 'main' character he will never die), and also the fact that the author really only has to flesh out Kurono, and to an extent Izumi, he can make them more dynamic, fuller beings. Kurono's growth, especially emotionally, isn't always on the up and up, but it waxes and wanes. Kurono is entirely human, because of the amount of time given to his character, and that in turn, I think makes for a good story in the case of Gantz.Besides the author could do what Akira Toriyama failed to do as he planned, and that was replace the main character with an entirely new one( Gohan was meant to become the new main character). It certainly allows the author more freedom to move about in his world, being as no one is free from the taint of death.

Wow, this was entirely way too long for a friendly rebuttal. =P

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G 5-
D 7-5h7- 7-7-5h7-5-0-
A
D 0-