James's Weezing evolved from his Koffing as James was complaining that he needed a stronger and more evolved Pokémon that could help him during battles. Weezing is usually sent out to use its Smog attack as a distraction or for combat cover. On many occasions, Smog has proved to be its most useful attack.
While in the Hoeun region, Weezing became attached to a group of Koffing that had been captured by a Pokémon Poacher. It was able to help them escape from the poacher and James saw how much it liked those Koffing and told it to stay there and watch over them. Initially, Weezing didn't want to leave but James was insistent and Weezing was released.