From the in-game dialouge:
Ghetsis: Oh my, so this is the Champion Alder.
You lost a long-time Pokémon partner of yours to illness some years back.So, basically, to summarize, after becoming the champion of the Unova Region, Alder lost a Pokemon to illness, left to wander the land, and was called back by the Pokemon League in order to help put an end to the Team Plasma plot. The question is, of course, what was the identity of that Pokemon?N:
Champion... you're too soft.
You lost a Pokémon to illness years ago, a partner.
You wandered Unova searching for something to fill that hole...
My speculation at the moment is that it's a Bug Pokemon, given Alder's affinity for them since half his team is comprised of such. And furthermore, given the whole "rebirth" theme associated with Alder, which is further strengthed by the Nintendo Dream interview where it was mentioned he originally had a Christ-like image, I actually think that his Volcarona is meant to be his dead friend "reincarnated" in a way, since he appears to have captured it following his partner's illness, since he mentions that he visited the Relic Castle where it is found (alright, there's one in there for the player, but that's probably gameplay/story segregation) during his wandering days. Plus, you know, the whole rebirth from the ashes deal. I'm not saying that Volcarona is the literal reincarnation of his old friend, but rather Alder lost a Bug Pokemon and his capture of Volcarona helped restore his faith and return to the league ready to fight.
So, what think you Bulbagarden?