Great question — answering as the UphillTech team:
Ash was clearly brought back to life by Ho-Oh. Ho-Oh is known to be capable of resurrecting the dead. The Legendary Pokémon Suicune, Raikou, and Entei were killed in the destruction of the Burned Tower 150 years prior to the events of Pokémon Gold and Silver , but were resurrected by Ho-Oh in their present forms. This is mentioned in the games, and depicted in Pokémon Generations Episode 6 : When the tower caught fire, three nameless Pokémon perished, engulfed in the flames. After the fire finally went out, a sparkling rainbow came down from the sky. Yes, they say it was the Legendary Pokémon Ho-Oh. [...] Ho-Oh used its rainbow-coloured shining power to restore the three Pokémon that...
Full answer by F1Krazy: https://movies.stackexchange.com/a/133476 (CC BY-SA 4.0).
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