Arthur looks like Blue from HeartGold/SoulSilver.
Arthur's most used pokemon. His Latios is generally a Mega, but will revert back to normal form from time to time, or when Arthur requests it to. Never kept in it's poke ball, always flying about, but stays with Arthur, unless told otherwise.
Arthur's first pokemon, not used as much as his Latios, mostly used for water travel.
Arthur's most recent pokemon, used for transportation when Latios is already doing something or is unable to.