She's just so much! Aaahh!!!
I feel like Duck is very much a very basic anime protagonist on the surface - clumsy but kind, willing to do anything for her friends, a little confused but she has the spirit etc. However!! While she may be basic, she is in no way boring!
I feel like a lot of the times clumsiness is slapped onto main characters for humor and cute value. And while it does make her silly and cute, it also plays a big role in her character and her struggles. She feels like she doesn't ever really fit in, clumsily navigating and trying to balance the life of Duck the girl and Duck the bird and her role in the story as Princess Tutu.
She's called dumb all the time, but she's very wise! Especially if you're a fan of Rue, you can see how wise Duck is when giving advice and talking to her about her feelings and their relationship with Mytho, even if she has trouble getting her point across. Her intentions, even if her words were jumbled, were felt and changed Rue for the rest of the series. Duck is very Empathetic and a lot better at reading people than she gives herself credit for. She thinks she only has the ability to "save" people as Tutu, but I think a lot of her greatest contributions to saving the others were done as Duck.
She also is very brave. She's the one who first examines her role in the story and tries to search for a better ending. I mean obviously, she's the protagonist, but still! Fakir had been going through the same song and dance his whole life, wishing to change the story and stop it from finishing, but he doesn't go actively looking for a better alternative, instead just sort of stalling for time by keeping Tutu from collecting the heart shards.
I really like how Princess Tutu handles it's characters and their struggles AS characters. I feel like a lot of the time when you hear about characters being "doomed by the narrative" or whatever it's referring more to characters who are doomed by action and not role. And due to being the protagonist, of course, this difference is shown best through Duck. She happily accepts her role as Tutu at first, but then later considers that by becoming human she has been losing her identity as a duck, and how the role of Princess Tutu was sort of forced upon her. Is she just Princess Tutu, or is she more?
And she worries about being a tool used for helping people, but still continues to help people regardless. She's not perfect obviously, but her heart is ALWAYS in the right place, and she is incredibly sincere and strong and I think it's great that she is still able to be the same silly girl while having to struggle with stuff like her identity, purpose and destiny.
I feel like a lot of the time when people apply "angst" to genki characters like Duck it can feel insincere. A lot of people see silly characters, ESPECIALLY GIRLS, as lacking depth. Even protagonists! But Duck is just so emotionally rich. She's very cheery and always wants to do her best, of course, but she's not tied to just being a happy character. Actually, she's probably the only one of the main four who doesn't really wear a mask! She doesn't need to, since her emotions and connections fuel the story and it's characters in a way that isn't just "Don't give up ^^ Love your friends ^^ Girl power!!"
She's awkward and loving and brave and a lot deeper than people (including herself, with all that "I'm just a duck" talk) give her credit for. She reminds me a lot of myself, stumbling blindly through social situations to find a connection and someone who could handle all the big huge emotions I have without being deemed unnecessary.
...I'm gonna be honest, I don't even know if a lot of the above makes sense. I sorta just rambled in a random direction until I felt I made a point lol. And I still feel like I didn't manage to say everything! Ahh!! I love this anime so much!!!