Before anyone says it, yes I know the trace ability cannot copy Zygarde's power construct ability in the games. The reason for this is that Gardevoir would be too powerful and cool looking if she turned into a Zygarde-naga in the games so they had to nerf the ability. (Source: my uncle works at nintendo as one of the technicians involved in Nurse Joy and Officer Jenny clone production, so he knows a lot about what goes on behind the scenes)

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Anyway, here we can see Gardevoir having copied Zygarde's ability, unintentionally causing Zygarde's cells ro rush to her aid and transform her! The very smol "2% forme" of Zygarde is none too pleased by this development, however, but fortunately for the upset little flatworm there are approximately 50% of the total cells still unaccounted for which should be plenty to combine with until Gardevoir can figure out how to change back, but who knows how long that could take... More pressingly, she should adjust to moving around with that big ol' Zygarde butt she's stuck with for the time being!