I think that the thing that impresses me the most about Ouverture is the way that they showed Suigintou's development physically. Each step is significant. First she only has competent control over her head and left arm. Then she graduates to floating (with some difficulty) through the N-Fields. Then Shinku teaches her how to walk, and at this point she's really gained some independance. However, this all crumbles after her encounter with Shinku and Souseiseki; but Rozen appears with her Rosa Mystica.
After this we see that she has full control over her body, despite her missing part. And then in her encounter we see her develop the two distinct traits that we recognize from the main series: her slit-shaped pupils and her wings. Her voice also changes notably as time passes, and kudos to Tanaka Rie for that. But, man, that moment when her wings first sprout is intense.