|Alternate Forms||Princess Tutu|
|Anime Debut||The Duck and the Prince|
|Japanese Voice||Nanae Katou|
|English Voice||Luci Christian|
Ahiru Arima, also known as Duck in English, is the protagonist of the series. She is a friendly, kind-hearted duck who was given a magic pendant that turns her into a human 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 is most commonly seen wearing her school uniform. She also wears a casual outfit consisting of a white turtleneck which shows her a bit midriff and has yellow folded frills in each side of the long sleeves, yellow cargo shorts with pockets resembling duck wings, and 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 along with her pendant.
Ahiru was an ordinary yellow duckling until she saw the prince dancing on the lake and saw how lonely he looked. Hearing her desire to bring him happiness, Drosselmeyer gifted her a pendant which gave her the ability to transform into a human girl and Princess Tutu. If Ahiru removes the pendant or quacks, 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. When Mytho was kidnapped by Rue as her alter-ego Princess Kraehe, Ahiru teams up with Fakir after she rescues him as Princess Tutu and this is how Fakir discovers both her human and duck form where he shows his embarrassment and disdain after seeing her naked body. During this time, she explains to Fakir about how she liked Mytho if he was handsome and seeing how lonely he was and this is her reason to transform into Princess Tutu to help him collect his missing heart shards. After rescuing Mytho from Princess Kraehe, she and Mytho helped the injured Fakir in a bon fire created by Edel.
In the second season, Ahiru is the only person can stop Mytho as the Prince of Crows when he attempts to steal the female's hearts as a sacrifice for the Raven as she transforms into Princess Tutu to convinced them about her dreams. She also informs Fakir about Mytho's change of personality due to the half of his heart shards inside of him were tainted from the Raven's blood and he was the only person can rescue her when she was damaged by the Ghost Knight as Princess Tutu in order to retrieve one of his missing heart shards or being captured and trapped by Drosselmeyer after Fakir was revealed to be Drosselmeyer's direct descendant who is able to write stories to become reality. In the near of the anime, Ahiru discovers Mytho transformed into a crow as well as Rue who is no longer became Princess Kraehe after breaking her sealed personality. She informs her that if Princess Tutu gives him five of his heart shards and the Raven will be revived. But because of her fate as a duck, she was told by Edel from her spirit that her pendant is actually the last of Mytho's heart shards. As soon as the Raven was revived and capturing Rue after showing her love for Mytho who is now reverted back to his human self, Ahiru as Princess Tutu watch Mytho fending off the humanoid crows until he was captured due to her pendant being stuck as Ahiru runs away but fall into the Lake of Despair and was rescued by Fakir who managed to break free from Drosselmeyer's control and the two made an intimate danced by performing the Pas De Dux and the necklace from her pendant was now loose, allowing Ahiru to keep the pendant. After waving goodbye to Uzura, Fakir and Autor in her human form, she transforms into Princess Tutu and gives the last of the heart shard to Mytho, turning her back to her duck form permanently as she watched Mytho transforms into his true self; Prince Siegfried while he summons his sword to fight the Raven. Ahiru is the only one can prevent Siegfried from shaterring his heart for the second time and convincing the townspeople who now transformed into crows to dance. But their dance performance causing Ahiru to get badly injured and to Fakir's horror and became deeply emotional to see her getting hurt. Ahiru does not giving up as she glows in a yellow light and the image of Princess Tutu can be seen to scatter in a white light to purify the townsfolk, allowing Siegfried to rescue Rue inside of Raven and both teamed up to slay the Raven once and for all. Ahiru was glad to see them okay before passing out due to her serious injuries as Fakir gets out on Autor's house to come to her aid. In the end of the anime she was seen swimming at the lake while Fakir is with her writing the new story.
- 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.