After Red had taken on Mewtwo in Pokémon Origins, he said that "Mew is still out there" and ran out of his house. This was the last we see of him. Or is it? In Pokémon Gold and Silver you can find Trainer Red atop Mt. Silver. He is the true final boss of the Generation 2 games in my opinion. But why is Red atop Mt. Silver? Here is my theory.
After Red starts his search for Mew, the first place he checks is Mt. Silver, which seems like the place where a legendary would rest. He climbs to the very top and to his despair, there is no Mew on Mt. Silver. Suddenly, without warning, a harsh blizzard starts, causing Red to be buried under about 6 feet of snow. Red tries to fight his way out, but eventually, his body temperature drops, and he freezes to death. Red's body is never found and he is eventually forgotten, despite his great deeds. Red waits atop Mt. Silver simply because he is a forgotten spirit, mourning over his long-lost legacy. He waits there, hoping for a worthy trainer to remember him, to challenge him for one last time, in an epic duel. Eventually, Red gets his wish and along comes Ethan/Lyra, the heroes from Gold/Silver. After the battle, you see Red disappear in the same blizzard that enveloped him, leading to his death. This is Red passing into the afterlife, as Mt. Silver was his eternal purgatory.
Hope you liked it.