In the past, Alder's starter Pokémon, died of illness, and in despair over not being able to save it, he began traveling around aimlessly. Before his partner's death, the two of them madly pursued strength, much in the way of Cheren. The identity of this Pokémon is never mentioned. However, it seems its death changed the way Alder looks at life. Also, Ghetsis's dialogue suggests that he may not have actually won the title of Champion, instead being requested by the Pokémon League to take the position.
In Pokémon Black 2 and White 2, Iris took over Alder's place of Champion. He can be found assisting the player at various points of the game. When visited at the Pokemon World Tournament, he reveals that his late partner was a Larvesta, which later evolved into Volcarona.
It was a Larvesta.