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For the games, she got both outfits or parts of them from Drayden. She has 3 different sprites across both games.(younger Iris in first game, then 2 more sprites in sequels). In the BW2 flashback you see Drayden gifts her the second outfit because she had the leggings so long.
But in BW2 when you run into her before the Champion fight she appears in her old outfit. So she still wears both I guess.
I feel like they did that just to keep it a "surprise" when you see her as Champion.
It's not actually a Champion uniform or anything though, so I assume she does wear it normally as well. She wears the outfit with sandals and it's a large dress so it's probably just not very practical to wear always, that's my headcanon for why she wasn't always wearing it.
