I always thought there was a little more than Ahiru being just a Nice Girl. She’s a duck. She’s not even human. Only such a simple minded pure creature could ever willingly take on such a role.

This is an interesting angle! A lot of philosophies consider animals to be completely innocent, since their actions are generally dictated by survival rather than malice. I don’t think this makes them necessarily good though, more like morally neutral. They don’t intend harm, but they don’t intend kindness either. Even if being a duck makes Princess Tutu inherently good, can this apply to Princess Kraehe, the crow? 

(This is very Ethics Lite and I haven’t seen the back half of the series yet, so I may very well be extremely wrong!)