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5. Si don't need to be *original or at canon time.* Si can be reborn say as god like izanagi, susanoo or Kagatsuchi ( who forms yomi faction to take revenge on izanagi) in ancient times, or some yokai, kami/ okami in mediaeval times , or 5 principal clan heir, or cao cao, vali, rizevim who wants to rule , Odin, heaven faction, tobio ikuse, mitsuyu kanzaki, even issei hyoudou (who remains human).
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5. Si don't need to be *original or at canon time.* Si can be reborn say as god like izanagi, susanoo or Kagatsuchi ( who forms yomi faction to take revenge on izanagi) in ancient times, or some yokai, kami/ okami in mediaeval times , or 5 principal clan heir, or cao cao, vali, rizevim who wants to rule , Odin, heaven faction, tobio ikuse, mitsuyu kanzaki, even issei hyoudou (who remains human).
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4. I feel you are *missing out* dxd different elements like youkai faction uplifting, human faction protection group, Shinto elements, inter pantheon politics , trade , culture, which are unexplored in dxd canon.