Wally lives with his parents in Petalburg City. He is shy, meek, speaks rarely, and seems to be very polite. With his partner, Ralts, he aspires to become a trainer and is inspired to catch up with you.
This inspiration will make him a strong rival. Ralts will eventually evolve to Gallade, and with a Key Stone outfitted in Wally's Mega Pendant, he can transform Gallade into Mega Gallade to battle.
Source: pokemonrubysapphire.com