Ponyo is the titular protagonist of Ponyo.
As a fish, she has a human face with short red hair and a red body. As a human, she has a short stature (being the size of a 5-year-old) with a red dress, red hair, and white bloomers.
Ponyo appears at the beginning of the movie with her sisters as they watch their father perform magic. She swims out into the ocean and ends up falling asleep in a jellyfish. Ponyo wakes up to a fishing boat bearing down on her and gets caught in the net. While struggling, she gets her head caught in a glass jar but manages to slip through the nets' holes. She tries in vain to shake the jar off and eventually collapses out of exhaustion close to shore.
Sosuke, a little 5-year-old boy who had come down to play with his toy ship, spots her and retrieves her. He too tries to get the jar off and eventually smashes it against a rock to break it. In the process, he cuts his thumb and it starts to bleed a little. Sosuke holds Ponyo and wonders if she is even alive when she suddenly licks the drop of blood off his thumb. He exclaims, "It licked me! It licked me!" At that moment, his mother, Lisa calls him in so he can go to school, and she to work. He scrambles up to his house, finds a plastic green bucket, and takes Ponyo with him in the car, but not before Ponyo's father tries to get her back. Him being from the ocean, he cannot step onto dry land without shriveling up unless he has some source of water, so he has fills a can that looks like weed killer and sprays everywhere he walks. Lisa thinks he's a "freakshow" and tells him to go away with his weed killer. He protests that is is not weed killer but the purest ocean water, but Lisa drives off before he can finish.
While they're in the car, Lisa hands him a sandwich and he proceeds to try and give Ponyo some of the bread. Ponyo turns her nose up at it and instead, grabs the ham, much to Sosuke and his mother's amusement/surprise. Lisa drops him off at Himawari Nursery School and proceeds to her job, which is right around the corner and happens to be a nursing home. One of Sosuke's school buddies, Kumiko comes out and shows off her new dress before realizing he has something. When she tries to see and Sosuke attempts to hide Ponyo, but she ends up seeing Ponyo anyways. When she does not go running for a teacher, Sosuke turns around and lets her see, but Ponyo turns her back on her. Kumiko is offended and calls Ponyo stupid, to which Ponyo whirls back around and squirts water in her face as well as dousing her new dress. Kumiko looks down at her dress and starts bawling and Sosuke takes that as a cue to run.
He ends up running right outside the nursing home his mother works at. He starts giggling with Ponyo and is heard by some of the elders. He shows them Ponyo and one of them claims that fish cause tsunamis. Ponyo squirts her and she starts thrashing and yelling "Tsunami!" Sosuke flees once again and goes down to the beach while his mother comes out looks for him, demanding him to come back and apologize. At that moment, Ponyo's father, Fujimoto, appears and sends two sea monsters to retrieve Ponyo. They successfully retrieve Ponyo, and all Sosuke can find is an empty green bucket. He starts crying and wades out even further into the ocean, desperately looking for Ponyo. Lisa spots him and rushes out and grabs him before he can drown. Fujimoto takes Ponyo back to her home in the ocean and seals her in a magical bubble. With the help of her sisters, she breaks free and makes her way to her father's sealed potion room where she gets into it and becomes human. She returns to Sosuke by running on huge waves in the form of fish along side his car as he is coming home from school. Sosuke sees her and watches in awe.
A huge storm breaks loose because, according to Fujimoto, the world is becoming unbalanced because of Ponyo. Ponyo ends up falling off and plummeting back into the ocean. Lisa stops the car at Sosuke's request, and they look for her, while Lisa tries to convince Sosuke that there was no little girl. As they pull into the driveway, Ponyo somehow is back, and she is holding Sosuke's green bucket. Sosuke and Lisa stare at her as she runs over to Sosuke and hugs him. This convinces Sosuke it really is her. Lisa takes them inside and starts to dry Ponyo off when she realizes Ponyo is completely dry. Sosuke explains it is because she is a fish. Ponyo begins performing magical deeds to help Lisa and wears herself out. Lisa prepares some hot noodles with ham for them, much to Ponyo's delight. Ponyo, having worn herself out from performing so much magic, falls asleep while eating. Lisa tucks her in and tells Sosuke to look after her. The storm continues to worsen, and when Lisa looks out the window, she sees the light on at the senior's home. She tells Sosuke she'll be back and leaves to go help.
In the morning, Sosuke wakes up to Ponyo in his face. He jumps up, and the two end up smacking heads. They look out and see that the storm was so bad, the whole city has been submerged in water. Since their house is on a high hill, they are the only ones who didn't get submerged. Sosuke wants to go find his mother since she has not yet come back and gets Ponyo to blow up his toy boat and make it his size. She also blows up a candle for him. While Sosuke is in the boat, Ponyo sits on top and is the lookout. They run into a large boat of people evacuating and are advised to hop on, but Sosuke refuses. They also run into a couple with a baby who is sick and unhappy. Ponyo gives them some soup Lisa made and swooshes her face into the baby's to make it laugh. The husband gives them a candle, and they depart.
Sosuke and Ponyo sail for a little while longer until Ponyo starts getting tired. The magic begins to wear off of the boat, and it becomes small again. Sosuke and Ponyo walk until they find Lisa's empty car. Sosuke starts crying, and Ponyo comforts him. The duo carries on until Ponyo starts shrinking. They are then trapped by Fujimoto, and he tries to get Sosuke to give him Ponyo. Yoshie, one of the elders, calls Sosuke over and tells him not to trust Fujimoto. Fujimoto grabs them and brings them down to the underwater garden he made, where everyone else, including Lisa, is.
The garden is enchanted and takes away all the senior's disabilities, allowing them to run around like children again. Lisa talks to Gran Mamare (Ponyo's mother) upon Sosuke, Ponyo, and Yoshie's arrival. Gran Mamare wakes up Ponyo and says she has to give up magic if she really wants to become human. She then puts Ponyo in a magic bubble and hands her to Sosuke, telling him he can choose. If he really loves her, he can kiss the bubble, and she will be human. If not, Ponyo will become nothing but sea foam. Upon returning to shore, Ponyo is impatient, bursts the bubble herself, and kisses Sosuke.
- She was most likely inspired by Ariel from Hans Christian Andersen's The Little Mermaid.
- Her fish form was inspired by the Ningyo, a legendary creature in Japanese mythology that is said to cause natural disasters when caught.