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I don't mean to bash her, but here are the reasons she is an advertising character
She is the main heroine in Ruby, Sapphire, and Emerald[R/S/E]
She competes in Pokemon contests, a new feature in Ruby, Sapphire, and Emerald[R/S/E]
She has two Pokemon from the first generation, which you can revisit in Fire Red and Leaf Green[FR/LG]
She has a future Pokemon from Diamond and Pearl, which of course is the fourth generation[D/P]
They brought her in for the show for the male audience, because the only thing a guy notices in a girl is her breasts :\ [ though technically, she doesn't have big breasts. In some episodes, she has small breasts. It all depends on which team is animating the episode.