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Given her name, I figured she'd have more to do with Diancie.
>DIANtha
>DIANcie
And Diantha's hair sort of resembles Diancie's crystal structures. I just knew that Diantha was going to be related to Diancie, somehow.
But she only appeared briefly in the Diancie movie, and has had nothing to do with Diancie in the anime. Not to mention that her Japanese name is Carnet, which has nothing to do with Diancie, really. It looks like her name was just a weird coincidence on the part of the localization team. Also Diantha was the worst Champion ever. Her team was so easy to beat, I don't even remember the fight. It was over just like that. Maybe in XY it was too easy to level up your mons, but beating her was child's play.
tl;dr Diantha got my hopes up for nothing in terms of Pokelore and she's a shit battler. Diantha a shit. Dianthafags a shit.