Rival May being silly and mostly non-threatening for your journey, only to jumpscare you after you become champion and challenge you for your title, without letting you heal first, and actually being a real threat, would be poetic. Especially if they play it off like perhaps she was only pretending to be non-threatening, to lull you into thinking she wasn't a serious trainer, collecting badges behind your back, fully intent on jumping up after you became champion. Just secretly a cold and calculating girl with a long-term plan.