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The first and most obvious aspect is of course her overdeveloped chest. That is to say, her big breasts.
Fanart has admittedly taken a run with her design, but not completely without reason. She had a more emphasized bustline compared to previous female leads during her first few anime appearances, and it came at a time where most fans were hitting puberty.
In recent times we also got a short (pic related), which further hints that she may actually have been intended as the titty girl among the female heroines on purpose. Even modern day characters like Lillie, Nenoma, Penny are deliberately designed the way they are with their respective audiences in mind.
Other factors are her design (hair/bandana), the iconic spats, or the fact that she was the first likeable or friendly rival after the douchebags of Gen I and II. For many young teenagers, I imagine, she was not only a sexual awakening, but also a first crush or gf. Me? I just like her breasts and fanart. A lot.
