<p>Although the anime movie version of the series was released after the Avatar series, the original version of the Buddha manga started in the 70's. With a cursory glimpse at the manga and the movies, I could tell Bryke was at least somewhat inspired by Osamu Tezuka's work. It's not just Siddhartha who seems a little like Aang, but Prince Virudhaka has a few parallels/contrasts to Prince Zuko as well, among other characters. Still, there were a few contrasts in character between Aang and Siddhartha (though Buddha has source material older than Tezuka's rendition). Given his character, I doubt Aang would care as much about overindulgence, for example.
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(Edited by Ashara Xerxes)