[XanderO]

Originally Posted by Ultimate PKMN Trainer Red

Uhh...you SEE him after that. You know he returns, right? It's a after-game. He returns to save Gardevoir so there ya go.

I never got to play after the main story, so no I didn't know about a mission with him. Still doesn't explain why he was in the netherworld with the main character.

Spoiler:

for those who don't know, Hydregon is revived later on and joins your rescue team.