|Alternate Forms||Princess Tutu|
|Anime Debut||The Duck and the Prince|
|Japanese Voice||Nanae Katou|
|English Voice||Luci Christian|
Ahiru Arima is the protagonist of the series, Or in English given the name of Duck. She is a friendly, kind-hearted duck who was given a magic pendant that turns her into a teenage girl. This pendant also allows Ahiru to turn into her alter-ego Princess Tutu.
Ahiru is a friendly, kind-hearted and goofy character. Like a duck, she is easily excitable, clumsy and talkative. She can be extremely scatterbrained and uncollected when nervous. As time goes on, she gains a strong determination and is more willing to take risks for her friends.
In the majority of her appearances, both human and non human, she wears Drosselmeyer's pendant. The pendant is a gold necklace with a red gem that rests in the center of her chest. In her human form, Ahiru has strawberry blonde hair that is braided with a huge cowlick, blue eyes, and light skin with freckles. She most commonly seen wearing her school uniform. In two episodes of the anime, she wears a casual outfit (a white turtleneck which also shows her midriff and has yellow folded frills in each sides of her long sleeves, yellow cargo shorts, white and red striped socks and red flats with a gold round at the center). The color scheme of her casual outfit resembles the color scheme of her duck form.
As a duck, she is yellow with blue eyes, with one single feather that sticks up at the top of her head.
During her ballet lessons, she wears a light grey leotard that has a dark blue stripe around the waist and a red jewel, similar to her pendant, on the front side of the neckline.
Ahiru was an ordinary yellow duckling until she saw the prince dancing on the lake and looking lonely. Hearing her desire to bring him happiness, Drosselmeyer gifted her a pendant which gave her the ability to transform into a teenage girl and Princess Tutu. If Ahiru removes the pendant or quacks while talking, she transforms back into a duck and must touch water while wearing the pendant to return to her human form.
Eventually she is able to harness the power of pendant to turn into Princess Tutu so that she can save Mytho. Due to this, she becomes a part of Drosselmeyer's story and will turn into a speck of light and vanish if she confesses her love to Mytho. Nonetheless, Ahiru continues with her quest to save Mytho by helping to collect his missing heart shards.
- Mytho: Ahiru developed a crush on Mytho when she saw him for the first time while he was dancing by a lake. Like most of the other girls, her crush was mild. As time passed, she and Mytho became close friends until he became possessed by the raven's blood. Despite all odds, she helps him conquer the evil Raven.
- Fakir: In the beginning of the series, there is tension between the two due to Fakir constantly interrupting conversations between Ahiru and Mytho. As the story progresses, Fakir and Ahiru develop feelings of affection for each other. The night that she tells him that she has no desire to fight with him, Fakir realizes that Ahiru is Princess Tutu. This realization is further reinforced when he sees her sneaking around the dorms shortly after the encounter and recognizes the pendant on her chest. When he confronts her about this, she admits that she is and feels relieved that he knows. Ahiru begins to have more positive and intimate thoughts about him after that. In the episode "Banquet of Darkness", Ahiru tells Fakir that she doesn't know why she likes Mytho and explained how she just thought he was handsome and had a desire to return the heart shards to him. She is also the one to make Fakir embarrassed and disdain after revealing herself in both of her forms as a duck and human. In this same episode, Fakir and Ahiru are seen swimming side-by-side, Fakir staring intently at their joined hands. Near the end of the series, it is revealed that Fakir is a direct descendant of Drosselmeyer. He has the power to write stories that come true, but is limited to only write stories about Ahiru. Fakhir continues to do so to save and protect her. Fakir initiates an intimate dance with Ahiru in the Lake of Despair and vows to stay by her side forever. He was also deeply emotional when Ahiru was badly injured while risking her life to save everyone who turned into crows. In the end, Fakir can be seen writing stories near in the lake while Ahiru is with him.
- Rue: Ahiru admires Rue for being an advanced student in ballet class and often tries to reach out to her as a friend. The first time Ahiru ever truly interacted with her was when they did a Pas de deux together after getting to know each other. When Rue became Princess Kraehe, Ahiru kept faith that the kind girl she knew was not lost forever.
- In the English Dub of the series, her name is Duck (which is the English translation of "ahiru").
- She is not a duck in the manga, although she is briefly turned into one.
- In the manga, her last name is mentioned being Arima.
- Her English voice actress, Luci Christian also previously voiced Hibari Hanakoganei from the manga and anime series, Cyberteam of Akihabara.
- Interestingly, both Ahiru and her alter-ego Princess Tutu are much sounded like Hibari's.