Quoted By:
I'm basing this more on the character herself than the rest of the cast.
Sylveon - Because it's cutesy and girly and has a similar color scheme to Princess Tutu's outfit. Also it's a fairy.
Spinda - Because Ahiru is a klutz and tends to get a lot of silly faces like that.
Purrloin - To represent Mr. Cat constantly threatening marriage toward her.
Kirlia - Looks a bit like a ballet dancer, also an "Emotion Pokemon."
Sunflora - For both her sunny disposition and because it can learn Petal Dance (and her signature move as Princess Tutu is "Flower Waltz")
Ducklett - Ahiru is a duck that turns into a girl that turns into a magical ballet princess/swan.