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Consider this: Koffing is FREE from the constraints of the real world. All he does is spit out gas onto his enemies, smile, and enjoy the fireworks. He suffers from NONE of the consequences of aiding his peers and is loved by a high number of trainers worldwide, to the point that the competitive scene's largest website is named after him. Can the same be said for Pikachu? No, Pikachu is an eternal slave to the dumbest trainer in the series, Ash Ketchum, who he has had to bail out multiple times from situations both grim and comedically simple. Ash never evolves Pikachu and, as a consequence, comes across as a one-off gimick that never advances in maturity. Koffing, however, evolves into Weezing, never having to think about the pain of a future evolution while also retaining its one destiny in life: gassing things.
Koffing should be the mascot.